Devlin was born in Cookstown, Co Tyrone to a Roman Catholic family. She attended St Patrick’s Girls Academy in Dungannon. She was studying Psychology at Queen’s University Belfast in 1968 when she took a prominent role in a student-led civil rights organisation, People’s Democracy. Devlin was subsequently excluded from the university.
She stood unsuccessfully against James Chichester-Clark in the Northern Ireland general election of 1969. When George Forrest, the MP for Mid Ulster, died, she fought the subsequent by-election on the “Unity” ticket, defeating the Ulster Unionist Party candidate, Forrest’s widow Anna, and was elected to the Westminster Parliament. At age 21, she was the youngest MP at the time, and remained the youngest woman ever elected to Westminster until the May 2015 general election when 20-year-old Mhairi Black succeeded to the title.
Devlin stood on the slogan “I will take my seat and fight for your rights” – signalling her rejection of the traditional Irish republican tactic of abstentionism (being absent from Westminster). On 22 April 1969, the day before her 22nd birthday, she swore the Oath of Allegiance and made her maiden speech within an hour.