Lucy Manvers in The Unloved was Windsor's first professional acting role. The Times described her character as "played with an unsettling stillness by Molly Windsor". She was discovered by the writer and director of The Unloved, Samantha Morton in a local drama school and casting agency, Rama Young Actors. She also had a role as Margaret's daughter in the recent release, Oranges and Sunshine.
Windsor was born in Nottinghamshire, where she lives with her family. Her brother is in the rock band Fahran. She attended the Nottingham Television Workshop, before switching to Rama Young Actors at the age of ten.