Throughout Leonard's working life, he was active in the Scottish Labour Party, including serving as election agent for Anne McGuire in Stirling at the 1997 general election where they unseated Secretary of State for Scotland Michael Forsyth by 6,411 votes. Leonard served as Chair of the Scottish Labour Party from 2002 to 2003 but opposed the 2003 invasion of Iraq, marching against the war and describing the bombing of Baghdad as "awful".