Shantaben had faced many financial hardships and she had hoped that her son would choose a more stable profession that would transform the family's status and fortunes, as opposed to the arduous world of cinema, which was highly dependent on connections, nepotism, and uncertainty. In a bid to dissuade her son and domesticate him, she decided to get him married at the age of 16. Kumar was completely confounded and although the engagement lasted several years (1954–1957), he refused to marry his chosen bride.

He later gained admission to Sashadhar Mukherjee's acting school, Filmalaya. Sadhana, Asha Parekh, Joy Mukherjee were amongst the other alumni from the school.

Sashadhar Mukherjee urged Sanjeev Kumar to join the Indian People's Theatre Association (IPTA). There, Kumar met A.K. Hangal, who was casting for his play, Damru. At the age of 19, Kumar played the role of a 60 year old man, a father of 6 children. Shaukat Kaifi, who played his wife, wondered how the young boy would play the role of an old man but upon seeing his performance she applauded his talent and effort, saying “Hari [Kumar] got a huge round of applause from the audience, the loudest one ever, and I was ignored and forgotten…He worked extremely hard on his sixty-year-old character, internalizing it so well that sitting, standing, talking and walking, he would look like a sixty-year-old band manager.(41)”

Sanjeev Kumar started appearing in Gujarati plays and made a name for himself there. But the struggle to enter the world of films was hard. Filmalaya Studios chose exceptional students from the Acting School and included them in small roles in their upcoming films. Thus, Kumar got his first film role in Hum Hindustani (1960).

A remake of the Bengali film Basu Poribar (1952), it was directed by Ram Mukherjee and starred Sunil Dutt and Asha Parekh in the lead roles. Although Kumar's role of a police inspector had no dialogues, he nonetheless gave it his best and seeing him on the silver screen gave his mother immense joy.

There are several harsh rejections in the life of an actor and Sanjeev Kumar's struggle was no different. After his role in Hum Hindustani, Kumar was ecstatic and excited at the prospect of working in other films. However, this proved to be an uphill task.

Kumar had heard that producer Tarachand Barjatya was looking to cast actors for his film Aarti (1962). Although Barjatya could not conceive of Kumar as the hero of the film, in large part due to his dishevelled appearance; Phani Majumdar, who was present at the meeting urged Barjatya to reconsider. Kumar was eventually cast as the lead and he was over the moon about the role. However, he was later replaced by Pradeep Kumar.

A few years later, Govind Saraiya, too, replaced him in Saraswatichandra (1968) as the director felt that Kumar's image did not fit the role.

Kumar tried to take these rejections and near misses in his stride but they nonetheless left an imprint in the actor's mind.

Although Sanjeev Kumar's big break remained elusive, he continued to get roles in plays and Gujarati films. Around the time when he was cast in the Gujarati film Ramat Ramade Ram (1964), he felt that Harihar Jariwala, his name, was not befitting for an actor. He thus adopted the screen name Sanjay Kumar and his films Ramat Ramade Ram and Aao Pyar Karen (1964), credit his work under this name.

However, when Sanjay Khan's film Dosti (1964) became a superhit, Kumar felt that two 'Sanjays' could not rule the screen at the same time, and altered his name yet again at the advice of Aspi Irani. Thus, Sanjeev Kumar was born.

A spate of lacklustre films followed as Sanjeev Kumar was cast in Nishan (1965) and Badal (1966), both of which were directed by Aspi Irani. Along with this, a few promising projects were shelved as well. To some extent, Kumar's working in B and C grade stunt films at this time, marred his chances with a few prominent directors and producers, as they failed to recognise his acting calibre.

However, it was H.S. Rawail's Sunghursh (1968) that allowed him to break the cycle. Starring Dilip Kumar, Vyjayanthimala, and Balraj Sahni, the film caught the attention of critics and filmmakers as Sanjeev Kumar managed to hold his own in front of the veteran actor, Dilip Kumar.

Sanjeev Kumar's big moment came with L.V. Prasad's Khilona (1970). Starring alongside Mumtaz and Shatrughan Sinha, Kumar played a man who loses his mind after witnessing a shocking incident. The film was a superhit, and Kumar was nominated for a Filmfare Award for Best Actor.

In the same year, he acted in Rajinder Singh Bedi's Dastak (1970), a milestone in his career. Based on a short story by Bedi, the film starring Rehana Sultan and Anju Mahendru, centres on a newly married couple who settle into an apartment once inhabited by a sex worker. Although the film did not fare well commercially, it was critically acclaimed, winning the National Film Awards for Best Actor for Sanjeev Kumar, Best Actress for Rehana Sultan, and Best Music Direction for Madan Mohan.

The 1970s saw Sanjeev Kumar firmly establish himself and gain renown for his versatility and willingness to take on unconventional roles.

In Gulzar's Koshish (1972), Kumar played the role of Haricharan, a differently abled (deaf and mute) character. The film portrays the conflicts, hardships, and struggles for a differently abled couple who try to establish a space for themselves within an indifferent society. The story was inspired by the Japanese film Happiness of Us Alone (1961).

The role required extensive preparation on Kumar's part and he was nervous about his performance. He said, “I was especially tense at the release of 'Koshish'—I sent a friend to the initial shows to watch reactions to the climax. The reaction had to be extreme. The audience would either hoot or clap. And they clapped. Now, whatever film I appear in, I know the characterisation will be compared with Hari in 'Koshish'.” (“I have only one competitor, Sanjeev Kumar”. Interview. Vijaya Irani. Sunday Magazine. 27 May 1973. Pg 20)

The efforts paid off when he won his second National Award for Best Actor for the film.

Throughout his career, Sanjeev Kumar balanced his off-beat roles and performances with more mainstream, commercial films. Addressing this dynamic in his career, Kumar said in an interview, “On the one hand the off-beat movies, which are big risks, have projected my true worth. On the other, there is the star-potential in the masala movies, which spell instant success.” (“I have only one competitor, Sanjeev Kumar”. Interview. Vijaya Irani. Sunday Magazine. 27 May 1973. Pg 20)

Ramesh Sippy's Seeta Aur Geeta (1972) cemented Sanjeev Kumar's mainstream success. Starring Hema Malini in a double role, the film featured Dharmendra and Sanjeev Kumar as the two love interests of the protagonists. The story uses a comedy of errors as its central device and although Kumar's role was not the central one, his comic timing was impeccable. The film was a major success, as were its songs.

During the filming of the song “O Saathi Chal”, Sanjeev Kumar and Hema Malini faced a life-threatening accident, narrowly missing death. The two actors, shaken by the incident, became close, laying the foundation for a personal relationship between the two.

Although Sanjeev Kumar excelled at comedy, seen in films like Raja Nawathe's Manchali (1973) and Shammi Kapoor's Manoranjan (1974), he continued to play a wide range of roles across genres.
Naya Din Nai Raat (1974) stands out in Kumar's oeuvre as he played a staggering 9 roles in the film. Corresponding to the 9 rasas in Indian aesthetic theory, the roles ranged from him playing an alcoholic, a wealthy widower, to a fake godman, a psychiatrist, amongst others. The role was initially offered to Dilip Kumar, who turned it down and the veteran actor later expressed his appreciation of Kumar's versatility and talent in performing the complex role. The film remained a feather in the cap of Sanjeev Kumar, who had delivered a landmark performance.

Directed by Gulzar, Aandhi (1974), starred Suchitra Sen and Sanjeev Kumar in the lead roles. The film centred on the romantic relationship between the protagonists, their separation, and the woman's subsequent engagement in politics. The film was a success upon its release but was banned after only 20 weeks of being exhibited in theatres. The ban was seemingly due to the film's violation of the Model Election Code of Conduct, claiming it could cause damage to the reputation of the Congress party. This was because the film was supposedly based on then Prime Minister Indira Gandhi.
Producer J Om Prakash approached the Minister for Information & Broadcasting at the time requesting for the ban to be lifted. The scenes depicting Sen smoking and drinking were removed and the film was subsequently re-released. Aandhi created quite a buzz and posters in some parts of the country even read, “See your Prime Minister on Screen.”
Kumar's performance was seen as being outstanding and he won the Filmfare Award for Best Actor for the film.

The film that perhaps Sanjeev Kumar is most remembered for, Sholay (1975), gave him the iconic role of Thakur. Even though the role was not that of a conventional hero, Kumar decided to take it on because of the powerful scenes that would allow him to command attention. Kumar had worked hard since a young age to modulate his voice, which was put into full effect while delivering the famed dialogues in the film.
Satyajit Ray was reportedly riveted by the film and watched two shows of the film on the same day, concentrating on the performances by Amjad Khan and Sanjeev Kumar, both of whom would be cast in Ray's Hindi film, Shatranj Ke Khiladi (1977).

Gulzar's Mausam (1975) starring Sanjeev Kumar marked yet another collaboration between the director and actor. The narrative centres on the unlikely, fraught relationship between a gruff bachelor and a young, foul-mouthed sex worker. A subtle exploration of people and relationships, the film unfolds as a meditation on regrets, loss, and the possibility of redemption.
The film starred Sharmila Tagore, and both lead actors were nominated for Filmfare Awards for Best Actor and Actress. The film ultimately won the Filmfare Awards for Best Film and Best Director.

Despite working alongside major popular stars, Sanjeev Kumar managed to hold his own and make his role a memorable one for audiences. Aap Ki Kasam (1974) was one such film where Kumar starred with superstar Rajesh Khanna.

This was observed again in Hrishikesh Mukherjee's Arjun Pandit (1976), where Kumar shared the screen with veteran actor Ashok Kumar, and Sanjeev Kumar won the Filmfare Award for Best Actor for the film.

Satyajit Ray, impressed with Sanjeev Kumar's acting in Sholay (1975), cast him as Mirza Sajjad Ali, one of the two noblemen obsessed with chess in Shatranj Ke Khilari (1977). He co-starred with Saeed Jaffrey, Shabana Azmi, and Farida Jalal. Ray took painstaking efforts to get everything right for the film and Javed Siddiqui was appointed to write the dialogues and to improve Kumar's enunciation of the Urdu words.
Although production was tentatively scheduled to start in January or February 1976; Kumar suffered a major heart attack. His illness, along with a car crash involving co-star Amjad Khan, caused an eight-month-long delay in shooting.
During the shoot, creative differences arose between Sanjeev Kumar and co-star Saeed Jaffrey. Kumar preferred to rehearse his lines and the shots multiple times before filming, while Jaffrey felt that excessive rehearsal hindered his spontaneity as an actor. The tension between the lead pair caused disruptions in the shoot and eventually, a solution was suggested by Richard Attenborough. Kumar rehearsed his shots with Siddiqui, while Jaffrey sat by himself during the rehearsals. Kumar saw the film as being a "once-in-a-lifetime opportunity" and gave his best for the role.

Yash Chopra's Trishul (1978) was another important film for Sanjeev Kumar, who shared screen space with Amitabh Bachchan, Shashi Kapoor, Hemal Malini, Raakhee, Waheeda Rehman. Although Kumar played Bachchan's father in the film, a supporting role in the ensemble cast, he insisted on being paid the same as Bachchan, owing to his status as a senior artist. The film was a big success and both Bachchan and Kumar were nominated for acting awards.
Curiously, when Kumar was a struggling actor, he had visited BR Chopra and Yash Chopra for a role in a film but was summarily dismissed by the brothers. Years later, they acknowledged that they had been hasty in their judgement and cast him in several of their films. Trishul was one of them.

Sanjeev Kumar continued to delight with his comedic performances, straddling them along with his serious roles. Pati Patni Aur Who (1978) directed by B.R. Chopra, and starring Sanjeev Kumar and Vidya Sinha, was a big success, once again highlighting Kumar's comic timing, and he was nominated for a Filmfare Award for the film.

Another multi-starrer where Sanjeev Kumar's acting prowess shined through was Silsila (1981). The story of the film was written by Sagar Sarhadi, who narrated a story about two young couples to Yash Chopra's wife, Pamela Chopra. Smita Patil and Parveen Babi were to play the female leads. However, Babi's health deteriorated, and Rekha replaced her. This is when Sanjeev Kumar suggested that Jaya Bachchan play the role of the dutiful wife, Shobha. Considering the existing rumours about Rekha and Amitabh Bachchan at the time, the film would create curiosity amongst audiences. Kumar convinced Jaya Bachchan to take on the role and she agreed, placing certain conditions of her own.

Despite the powerhouse of talent in the film, Kumar's performance stood out, especially in the song, “Rang Barse”, where he added nuance to his role in the overall story. Although the film did not fare very well at the box office, it gained momentum years later and is considered a classic today.

In the same year, Kumar also acted in Raj Kapoor's comedy Biwi-O-Biwi. Directed by Rahul Rawail, Kumar played a double role in the film.

Another collaboration between Gulzar and Sanjeev Kumar, Angoor (1982), was based on William Shakespeare's play, The Comedy of Errors. The film narrates the tale of two pairs of twins separated in childhood, who reunite as adults, setting off a series of humorous confusions involving spouses, masters, and servants. Kumar played a double role, along with Deven Verma. Much of the film's success is attributed to Kumar's performance and Gulzar's punchy, witty dialogues.

The collaboration between Gulzar and Sanjeev Kumar was one that enabled both artists to fully explore unconventional subjects and roles, while realising their own ambitions as a filmmaker and actor respectively. Namkeen (1982), starring Kumar, Sharmila Tagore, Shabana Azmi and Waheeda Rehman; was another film on an unusual story about three women, and their strict mother.

The shooting of the film was plagued by several disruptions and difficulties. Rekha was initially chosen for the film but refused to shoot as she had some grouse with freelance journalist H Khatiuriya, who was present on the sets. Gulzar chose to side with his friend, which resulted in the actress leaving the film. Sharmila Tagore subsequently stepped in.
Another issue was the tardiness of Sanjeev Kumar, who was the only member of the film who would come late to the sets, causing other members to complain about him.

Nonetheless, the film won critical acclaim and received the National Film Awards for Best Audiography for Essabhai M. Suratwala. It also won Best Story and Best Art Direction at the Filmfare Awards.

Sanjeev Kumar faced several setbacks in his personal life. After his relationship with Hema Malini ended, he decided never to get married. He fell in love with Nutan, who was facing marital conflicts at the time. However, that relationship did not come to fruition either. Kumar was heartbroken and did not marry. He was known for sitting with his friends every evening and drinking late into the night, which ultimately took a toll on his health.
He suffered a couple of heart attacks and underwent a bypass surgery in the US, even quitting smoking thereafter. He died tragically early at the age of 47, on 6 November 1985.

A number of films starring Sanjeev Kumar were released after his demise. This includes Haathon Ki Lakeeren (1986), Qatl (1986), Baat Ban Jaye (1986), Namumkin (1988), Professor Ki Padosan (1993) amongst others. However, the most significant one of them was Love and God (1986), a film that had a troubled production history since the very beginning.

The original star cast featured Guru Dutt in the lead role as Kais, with Nimmi cast as Laila. The shooting began in 1963 but Dutt's sudden death in 1964 left the film incomplete and it was shelved. Later, K Asif cast Sanjeev Kumar as Kais and the production of the film was resumed in 1970. However, K. Asif passed away in 1971, leaving the film incomplete once again. 15 years later, Asif's wife, Akhtar Asif decided to piece the film together and it was finally released in 1986. By this time, Sanjeev Kumar too, had passed away.

Sanjeev Kumar left a rich legacy behind through many of his roles, which remain etched in popular memory. His versatility, comic timing, and ability to portray nuanced characters in a range of films remains unsurpassed. Starting from stunt films at the beginning of his career, to sharing the screen with the top acting talent in the country, Kumar had a meteoric rise and remained true to his craft, choosing roles on their potential alone. He would work for hours on his make-up and dialogue delivery to ensure that every character was portrayed in its truest manner.

Aside from his talent as an actor, Kumar is remembered for his grounded personality, which endeared him to many of his co-stars.