Mitra was born on 15 August 1900 in Kolkata, in a middle class Bengali Kayastha family.

He passed matriculation from Hindu School, Kolkata in 1915 and became graduate from the University of Calcutta in 1919.

He founded the Swaraj Sevak Sangha and was attached with Hooghly Vidya Mandir which was headed by Bhupati Majumder, one of the Jugantar Leaders in 1922.

[1][2] On 16 September 1931, police shot and killed Santosh kumar Mitra and another inmate Tarakeswar Sengupta in Hijli Detention Camp.

[3][4] Santosh Mitra Square, the famous park in Kolkata is named after him.