In the 1950s, Rajakumar was active in the Socialist Club while studying medicine at the Singapore campus of University of Malaya where he met historian Khoo Khay Kim and former Parti Sosialis Rakyat Malaysia Kassim Ahmad, who were also students.
Politically, his influence waned after the decline of the Barisan Socialis in the late 60s but he always held on to his ideals.