He joined the Scottish Liberal Party in 1980 and was the election agent for Veronica Crerar when she won the South East Corstorphine ward in 1984. He was also the election agent at council and parliamentary elections for Donald Gorrie. John Barrett was elected to the City of Edinburgh Council for South-East Corstorphine ward in 1995 and was elected Chairman of the Liberal Democrat Group from 1995 to 2001. He was the election agent for Donald Gorrie in Edinburgh West when the Liberal Democrats won the seat from the Conservatives at the 1997 General Election.