Sukhdev Thapar was a member of the Hindustan Socialist Republican Association (HSRA), and organised revolutionary cells in Punjab and other areas of North India. He also taught at the National College in Lahore, where he and other revolutionaries also established the Naujawan Bharat Sabha, an organisation involved in various activities intended mainly to prepare youths for the struggle for independence and putting an end to communalism.
Sukhdev is best remembered for his involvement in the Lahore Conspiracy Case of 18 December 1928 and its aftermath. He was an accomplice of Bhagat Singh, and Shivaram Rajguru, whose conspiracy led to the assassination of Deputy Superintendent of Police, J. P. Saunders in 1928 in response to the violent death of a veteran leader, Lala Lajpat Rai. After the Central Assembly Hall bombings in New Delhi on 8 April 1929, the conspirators were arrested and convicted of their crime.