Merron was made a prospective parliamentary candidate (PPC) through an all-women shortlist, and was elected to the House of Commons in May 1997 with a majority of 11,130. From 1997 until 2007, when Quentin Davies defected to the Labour Party, she was Lincolnshire's only Labour MP - and the first since Margaret Beckett had the seat in 1979. In the 2005 general election, her majority was 4,613. She lost her seat to the Conservative candidate Karl McCartney in the 2010 general election.