Mary Foy was born in Jarrow, County Durham, and grew up on a council estate. She is the second of five children and her grandparents were Irish immigrants. Her father is a former shipyard worker. Foy has a degree in social sciences, which she gained as a mature student.

Mary is a member of both UNISON and Unite the Union. She was a Community Development Worker in Durham from 2006 to 2013, and she also worked as a parliamentary assistant to former Jarrow MP Stephen Hepburn.

She was elected as a local Councillor to represent the Lamesley ward on Gateshead Council Metropolitan Borough Council in 2006. She later became the cabinet member for health and wellbeing on the council in 2009, holding the post until she stood down from the council in 2019. She was local party chair for Blaydon before moving to the City of Durham, and a regional representative of Labour’s National Policy Forum.

She stood down from Gateshead Council when elected as an MP for City of Durham in 2019, and joined the Socialist Campaign Group parliamentary group.

In November 2020, Mary was involved in the saving of a local school, St Thomas More Primary School, Belmont. Foy had met with representatives of the diocese and the governing body, Durham County Council officers and elected officials, parents and the local community to listen to concerns and to help find a way to resolve the situation.

Mary was part of the Committee which scrutinised the Health and Care Bill in Parliament. As part of this committee, she tabled several amendments relating to smoking health, including banning a loophole which enables vapes to be handed out for free to underage people and raising the age of sale of cigarettes to 21.