Ashok Kumar was born Kumudlal Ganguly to a Bengali family in Bhagalpur, then in the Bengal Presidency of British India and now in the Bihar state of India. His father, Kunjlal Ganguly, was a Lawyer while his mother, Gouri Devi, was a home-maker. Kumudlal (as he was then known) was the eldest of four children. His only sister, Sati Devi, a few years younger to him, was married at a very young age to Sashadhar Mukherjee and became the matriarch of a large "film family". Next was his brother, Kalyan, more than 14 years younger (b.1926), who later took the screen name Anoop Kumar, and youngest of all was Abhas (b.1929), whose screen name was Kishore Kumar, who became a phenomenally successful playback singer of Hindi films. Although the eldest by several years, Ashok Kumar outlived all his siblings. In fact, he stopped celebrating his birthday after his youngest brother, Kishore, died on that very day, Ashok's birthday, in 1987.

Ashok Kumar's sister Satidevi was married at a very young age to Sashadhar Mukherjee, who lived in Mumbai and worked as a technician in the film industry. This connection resulted in Ashok Kumar becoming somewhat interested in the technical aspects of film-making (not of acting). He failed his law exams and, to escape acrimony at home, came to live with his sister for a few months, until the exams were held again. In order to earn some spending-money for himself, he requested his brother-in-law to find him a job. Sashadhar Mukherjee was working in a fairly senior position in the technical department of Bombay Talkies, a pioneering Indian film studio, and he used his influence to get Kumudlal (Ashok's real name) a job there. Kumudlal started off as a laboratory assistant in Bombay Talkies and lived with his sister's family in Chembur, not far from the studio. This was in the early 1930s. The salary was fairly decent; furthermore, Kumudlal was successful at his job and enthusiastic about it, which had not been the case with law college. He managed to convince his father that he would not become successful as a Lawyer and would be able to earn a living as lab assistant. His father finally reconciled himself to the situation and granted permission to Kumudlal to abandon his law studies. Kumudlal thus began his film career, albeit as laboratory assistant in a film studio.

The two did a string of films thereafter, including Janmabhoomi (1936), Izzat (1937), Savitri (1937), Vachan (1938) and Nirmala (1938). Their last on-screen venture was the 1941 movie Anjaan, whose failure at the box office brought an end to the legendary on-screen couple. Devika Rani was consistently the bigger star, with Ashok Kumar working in her Shadow.

He started emerging from Devika Rani's considerable Shadow owing to pairing opposite Leela Chitnis, another Actress who was senior to him in age as well as stature. Back-to-back successes with Kangan (1939), Bandhan (1940) and Azad (1940) saw Ashok Kumar emerge as a popular actor in his own right. The success of Jhoola (1941), in which he starred opposite Leela Chitnis, established him as one of the most bankable actors of the era.

The Gyan Mukherjee directed 1943 movie Kismet, featuring Ashok Kumar as the first anti-hero in Indian Cinema smashed all existing box office records, becoming the first Hindi movie to gross 1 crore at the box office. The success of Kismet made Ashok Kumar the first superstar of Indian cinema. Such was his popularity at the time that, in the words of Manto, "Ashok's popularity grew each passing day. He seldom ventured out, but wherever he was spotted, he was mobbed. Traffic would come to a stop and often the police would have to use lathis to disperse his fans."

After Kismet, Ashok Kumar became the most bankable star of the era, delivering a succession of box office successes with movies such as Chal Chal Re Naujawan (1944), Shikari (1946), Sajan (1947), Mahal (1949), Sangram (1950) and Samadhi (1950).

Ashok Kumar is also credited with mentoring several personalities who went on to make significant contributions to Indian cinema. As Producer with Bombay Talkies, Ashok Kumar gave Dev Anand his first break in Ziddi (1948), which also established Pran (then a struggling actor who had just fled to India during partition), as one of the leading villains of the era. The 1949 film Mahal, starring Ashok Kumar and made under his watch at Bombay Talkies launched the career of Madhubala, one of the leading actresses of the 1950s. The legendary song "Aayega Aanewala" from Mahal was the turning point in the career of a hitherto little known young singer called Lata Mangeshkar.

Ashok Kumar appeared frequently opposite Nalini Jaywant (with whom he was rumoured to have had an affair), in several movies of the 1950s. He did around 17 films with Meena Kumari in a span of twenty years ranging from Tamasha in 1952 to 1972 magnum opus, Pakeezah. He played the suave cigarette-smoking Criminal or police officer in several films in the mid to late 1950s, in what was the Indian film-noir movement.

Off the screen, Ashok Kumar gave B.R. Chopra, then a film critic and unsuccessful filmmaker, his first break as Director with the 1951 film Afsana. The success of Afsana established Chopra as a respected filmmaker. Ashok Kumar also played mentor to his assistant at Bombay Talkies, Hrishikesh Mukherjee, who went on to become one of the great Directors of Hindi cinema. He was the lucky mascot for a promising young Director called Shakti Samanta in the late 1950s, delivering a series of hits with Inspector (1956), Howrah Bridge (1958) and Detective (1958) which helped the young man establish himself as a successful Director. Shakti Samanta would go on to deliver several movies in the 1960s and 1970s which are regarded today as classics.

Ashok Kumar also paved the way for his younger brothers Kalyan (Anoop) and Kishore Kumar. While Anoop is best remembered for his role in Chalti Ka Naam Gaadi (1958), Kishore went on to become a legendary singer. Arguably, Kishore is today the most popular of the brothers.

Ashok Kumar played important roles in several landmark movies in the 1960s and 1970s, including Jewel Thief (1967), Aashirwad (1968) (for which he won a Filmfare Award as well as National Award in 1969), Purab aur Pashchim (1970), Pakeezah (1972), Mili (1975), Chhoti Si Baat (1975) and Khoobsurat (1980).

While still a teenager, and well before he had even given thought to a career in films, the young Kumudlal was married to Shobha in a match arranged by their parents in the usual Indian style. Their lifelong marriage was a harmonious and conventional one, and despite his film career, the couple retained a very middle-class outlook and value system, bringing up their children with traditional values in a remarkably simple home. They were the parents of one son, Aroop Ganguly, and three daughters named Bharati Patel, Rupa Verma and Preeti Ganguly. Their son, Aroop Kumar Ganguly, worked in exactly one film, appearing as hero in Bezubaan (1962), which flopped at the box office. He then made a career in the corporate world. Ashok's eldest daughter, Bharati Patel, is the mother of the Actress Anuradha Patel, who is married to actor Kanwaljeet Singh. Ashok's second daughter, Rupa Verma, is the wife of the actor and Comedian Deven Verma. Ashok's youngest daughter, Preeti Ganguly, was the only one among his daughters to enter the film industry. She acted as a comedienne in several Hindi films during the 1970s and 1980s, and died unmarried in 2012.

He acted in fewer films in the 1980s and 1990s, and occasionally appeared on television, most famously anchoring the first Indian soap opera Hum Log and appearing as the title character in the unforgettable Bahadur Shah Zafar.

Ashok Kumar's last film role was in the 1997 movie Aankhon Mein Tum Ho. Besides acting, he was an avid Painter and a practitioner of homeopathy. A qualified homoeopath, Ashok Kumar earned a reputation for conjuring up miracle cures. Altogether, he starred in over 275 films. He has done more than 30 Bengali dramas in Dhakuria.

Ashok Kumar died at the age of 90 in Mumbai on 10 December 2001 of heart failure at his residence in Chembur. The then Prime Minister Atal Bihari Vajpayee described him as "an inspiration... for many generations of aspiring actors."