Osborne was the councillor for Whitletts on Kyle and Carrick District Council and South Ayrshire Council where she also served as Convener of Community Services (Housing and Social Work). She is a member of the trade union Unite (formerly TGWU) and has been active in the Labour Party since 1976, serving for a time as Ayr CLP Secretary. In 1983, she was a member of the Scottish contingent on the People's March for Jobs who walked from Glasgow to London. She was selected as a Labour candidate in Ayr from an all-women shortlist. In May 1997, she was elected as MP for the Ayr constituency, becoming Ayr's first-ever Labour MP and first ever female MP. She was re-elected at the 2001 general election.