McIsaac started her political career as a councillor to the London Borough of Wandsworth in the Tooting ward in 1990. She was selected to stand for election for Labour in Cleethorpes through an all-women shortlist. She won the Cleethorpes seat with a majority of over 9,000, defeating Michael Brown, the sitting MP for the predecessor seat of Brigg and Cleethorpes. She was re-elected in 2001 with a majority of 5,620, and again four years later, finishing over 2,000 votes ahead of her Conservative rival.