Kumar first appeared on the silver screen as an uncredited background artist in 1931, and got his first leading role in Bombay Talkies' Jeevan Naiya (1936). In 1938, he starred in Achhut Kanya and Achyut. He established a prolific screen partnership with his younger brother Kishore Kumar in such films as Jeevan Prabhat, Bandhan, Sharada, and Samadhi.
After achieving success in romantic and social dramas, Kumar began starring in action and stunt-based films beginning in the 1950s, such as Sangdil, Police and Ziddi. In the 1960s, he made his directorial and production debut with Black Mail (1965) for which he won a Filmfare Best Director nomination. As an actor, he received acclaim for his performances in classics such as Mahal, Aashirwad, Mashaal, and Rakhwala. He remained active in the industry until his death in 2001, appearing in his final film, Aankhon Mein Tum Ho, in 2002.
Kumar was the recipient of several accolades, including a Dadasaheb Phalke Award—India's highest award in cinema—in 1988, and a Filmfare Lifetime Achievement Award in 1995. He was also honored by the Indian government with Padmashri, the fourth-highest civilian award, in 1990. In addition to his film work, Kumar is known for his autobiography, "Ashok Kumar: My Life", which was published in 1987.