Hattersley was a socialist and Labour supporter from his youth, electioneering at the age of 12 for his local MP and city councillors, beginning in 1945.
At university Hattersley joined the Socialist Society (SocSoc) and was one of those responsible for changing its name to the "Labour Club" and affiliating it with the non-aligned International Union of Socialist Youth (IUSY) rather than the Soviet-backed International Union of Students.