Montgomery was born at her family's ancestral home in Greyabbey, County Down, the daughter of william Howard Clive Montgomery, OBE, of Rosemount House and of Greyabbey, and his second wife, Daphne Bridgeman. Her maternal grandfather was Geoffrey Bridgeman. She is a descendant of the 1st Viscount Montgomery. She was educated at Rockport School, County Down, and Downe House School, Berkshire. She then studied drama at The Gaiety School of Acting in Dublin, Ireland. She graduated in 1994.
Montgomery also appeared in an episode of the medical series "Body Story 2" in 2001. In the episode, she played a woman named Phoebe Hamilton. The episode showed what happened to her character's body in response to a wasp sting. In 2012 she had a role in the pilot episode of ITV's Endeavour.
In 2003, she was named as one of European film's Shooting Stars by European Film Promotion. Her notable TV appearances include The Bill as Emma, Urban Gothic, Pulling Moves and Midsomer Murders. She also acted in the 2001 film The Discovery of Heaven and in 2006 in Basic Instinct 2.
