Kumar's early films were flops, and he was nicknamed "Flop Master General". In 1952, Kumar had a supporting role in Basu Poribar, directed by Nirmal Dey, which was his first role to receive praise. The following year, he worked with the same studio and director in the film Sharey Chuattor, which was his first time paired with Suchitra Sen. His first breakthrough role was in Agradoot's film Agnipariksha in 1954. Collaboration with Satyajit Ray Satyajit Ray originally approached Kumar to appear in his film Ghare Baire, which he wrote in 1956. Ray approached Kumar to appear as the role of Sandip, but Kumar refused, believing the role would be better for an established actor. Ray completed the film in 1984, after Kumar's death, casting Soumitra Chatterjee for the role. Kumar collaborated with Ray for the first time in 1966 on the film Nayak. After seeing Kumar in Nayak, actress Elizabeth Taylor was impressed with his performance and wanted to work and meet with him. In 1967, Kumar worked with Ray in Chiriyakhana. When the Government of India instituted National Film Awards, Kumar was the first actor to be honored with the National Award for Best Actor at the 15th National Film Awards in 1968 for his performances in Chiriyakhana and Anthony Firingee. Producing and directing Kumar produced six Bengali films and one Hindi film. The first films he produced, Harano Sur (1957) and Saptapadi, were under the banner of Alochayas Production. Both films were directed by Ajoy Kar and received the National Film Award. In 1963, he changed the name of his production house to Uttam Kumar Films Private L.T.D. The first film produced under this banner was produced Bhranti Bilash, which was followed by Uttar Falguni, Jatu griha, and Grihadaha. Later, Kumar also directed the films Sudhu Ekti Bochhor, Bon Palashir Padaboli, and Kalankini Kankabati (released after his death). He composed music for the film Kaal Tumi Aleya, in which Hemanta Mukherjee and Asha Bhosle sang. and also composed Sabyasachi (1977). He was also a playback singer in his own film Nabajanma (1956). Later he also made a recording singing Rabindra Sangeet. He first sang in the 1950 film Maryada. Theatre Kumar acted in theatre before debuting in the film industry. In 1953, he returned to theater, acting in a play called "Shyamali" under the Star Theater banner. Shyamali ran for over 480 nights. The director Ajoy Kar made