Reid was born in Whitstable, Kent, England. Her father was a doctor in the British Army. She lived in Germany, Africa, and the United States before settling in Canada with her family in 1964.
She studied acting at McGill University, receiving a Bachelor of Arts degree in 1972, and at the Banff Centre for the Arts.
Reid's first professional acting job was a school tour with the Globe Theatre in Saskatchewan.
She appeared for three seasons as Cathy on the King of Kensington and was nominated in 1977 for an ACTRA Award for Best Performance in a Continuing Role in a Television Program. She left after expressing a desire to return to the stage.
She has been a mainstay on the stage at the Shaw and Stratford Shakespeare Festivals.
Reid voiced Mrs. Jenkins in the animated series Timothy Goes to School.
In 2009, she starred as Sister Aloysius in Doubt, a Parable at the Neptune Theatre in Halifax, Nova Scotia.
In 2011, she starred as Violet in August: Osage County at the Citadel Theatre in Edmonton, Alberta.
In 2017, Reid stars as Queen Elizabeth II in the Mirvish Productions presentation of The Audience at the Royal Alexandra Theatre in Toronto. The role was originated in the West End and on Broadway by Helen Mirren.
She has twice won the Dora Mavor Moore Award, in 1993 for Fallen Angels and in 1995 for Six Degrees of Separation. She won an Elizabeth Sterling Haynes Award (Sterling Award) for her performance in August: Osage County.
In 2006, she was made a Member of the Order of Canada She received The Barbara Hamilton Award in 2008. and an award of excellence from ACTRA Toronto in 2010.
She was awarded an honorary degree from Bishop's University.
Reid met McCowan Thomas doing summer stock theater and married him in 1977. They have two children, Alec and Julia.