Riddell served as executive secretary of the LSA from 1972 until 1977 when it merged with other Trotskyist groups to form the Revolutionary Workers League/Ligue Ouvrière Révolutionnaire.
In 1972, Riddell “was the target of an extensive campaign of [anonymous] letters containing various attempts to discredit him.”[8] In 1979, Royal Canadian Mounted Police officers testified to an Ontario Public Inquiry that they had written and distributed these attacks, based on false information.
[9] Riddell then took part in legal initiatives by Harry Kopyto and Ross Dowson to secure redress[10] in “a series of high-profile, but unsuccessful, lawsuits against the Royal Canadian Mounted Police” with reverberations lasting until 2016.
[11] In 1983, Riddell began work with the Socialist Workers Party and Pathfinder Press on the Communist International publishing project,[12] relocating to New York City in the mid-1980s.
[13] He returned to Toronto in 1994, and was active with the RWL's successor, the Communist League until 2004 when he broke with the CL and the SWP over their lack of support for demonstrations against the 2003 invasion of Iraq.