He returned to India and started working in R. G. Kar Medical College and Hospital in Calcutta as Resident Surgeon and Assistant Superintendent.

He became the director of Chittaranjan Seva Sadan and Principal of its College of Obstetrics, Gynecology and Child Health, Calcutta.

He built Chittaranjan Cancer Hospital in 1950 with handsome donation to Deshbandhu Memorial Trust in Calcutta.

The technique developed by Dr. S. Mitra for Cancer Cervix Operation namely Extended Radical Vaginal Hysterectomy with Extraperitoneal Lymphadenectomy was officially announced in British, American and German gynecological conferences.

This has been published in a monograph "Mitra Operation for Cancer of the Cervix" simultaneously by Charles C. Thomas, U.S.A. and Ryerson Press, Toronto and Blackwell Scientific Publications, Oxford.