After unsuccessful forays into Ontario provincial politics, LaMarsh was first elected to the House of Commons of Canada in a 1960 by-election. Sitting as a Liberal Member of Parliament (MP) for Niagara Falls, she joined the Canadian Cabinet after the Liberals defeated the Progressive Conservative government of John Diefenbaker in the 1963 election. Serving under Prime Minister Lester Pearson, she was the second female federal cabinet minister in Canadian history, and the first to serve in a Liberal cabinet. LaMarsh served as Minister of National Health and Welfare and Minister of Amateur Sport from 1963 to 1965, and as Secretary of State for Canada from 1965 to 1968.