After his Ph.D., Mitra went to Cornell University, where he completed a second Ph.D., working with Freeman Dyson and Hans Bethe. Mitra returned to India in 1955, and was appointed Reader at Aligarh Muslim University. He moved to Delhi University as professor in 1963, where he remained until his superannuation in 1994. Mitra was head of the Department of Physics and Astrophysics during 1973–75, but stepped down without completing his three-year term. He held visiting appointments at Indiana University (1962–63) and at the University of Illinois at Chicago (1986–87). He held the prestigious chair of INSA: INSA-Albert Einstein Research Professor during 1989–94 at the University of Delhi. He was a life-time emeritus professor of the university of Delhi at its department of physics and astrophysics. He has produced many PhD's who now occupy leading academic positions.