As a young man, Cannon was a keen supporter of the Communist Party. From 1948 to 1954, he was an executive councillor in the Electrical Trades Union (ETU). he left the Communist Party in 1956, disillusioned by the Soviet invasion of Hungary.
He subsequently began campaigning against Communist attempts to rig ETU elections. This led to a high-profile trial in 1961, that resulted in the resignation of the ETU General Secretary.
Cannon died in 1970, shortly before he was awarded a knighthood.
- Papers of and relating to Sir Leslie Cannon (1920-1970), trade union leader, Modern Records Centre, University of Warwick, accessed 7 November 2012.