In 1974 she was elected as Labour MP for Northampton North. She appeared to be a happily married woman with three grown up children, but she soon left her husband to set up home with a female partner, magazine publisher Barbara Todd, thereby becoming Britain's first openly lesbian MP. This came as a shock to the Labour Party. As her local party chairman Norman Ashby said:
- "She was elected as a working wife and mother... this business has blackened her image irredeemably."
She was deselected as candidate for the seat by the local Labour association. This was reversed by the national party, but she lost the seat to the Conservatives at the 1979 election.
She served on Hackney Borough Council from 1982 to 1990 and, after moving to the Lake District, on the National Park authority there between 1998 and 2006.
She died 2 February 2021.
Colquhoun features in a book by Helen Lewis called 'Difficult Women: A History of Feminism in 11 Fights' published by Jonathan Cape in 2020 . In February 2021 Lewis was due to attend an on line celebration of Colquhoun's life hosted by LGBT Labour (Labour Party) but the invitation was withdrawn and the event cancelled because Lewis had views at variance with the group on gender identity .