Rennard was educated at the Liverpool Blue Coat School and gained a BA (Hons) Social Studies in 1982 from the University of Liverpool. From his early teens he was an active member of the Liberal Party. He began his political activism in Liverpool in the late 1970s, where he chaired the Liverpool University Liberals. He cites the early pioneers of community politics in Liverpool, including Cyril Carr, Trevor Jones and David Alton, as having had a major influence on him. Rennard was Deputy Chair of the Liverpool Liberal Party and organised many of the party's election campaigns in the 1980s, at a time when the Liberal Party controlled the City Council. The first successful parliamentary by-election campaign he worked on was the Liverpool Edge Hill by-election in March 1979, at which the party's candidate was David Alton. Rennard was Alton's agent when he successfully contested the newly created Liverpool Mossley Hill constituency in 1983 (after boundary changes) and helped to secure an excellent result there in a general election that saw a landslide Conservative victory nationally.