Foy was born in Stockport, and she grew up in Manchester and Leeds, the youngest of three children. Her family later moved to Longwick, Buckinghamshire. She attended Aylesbury High School, a girls' grammar
school, from the age of twelve; she then attended Liverpool John Moores University, studying drama and screen studies. She also trained in a one year course at the Oxford School of Drama. She graduated in 2007 and currently lives in Peckham "with five friends from drama school".
While at the Oxford School of Drama, Foy appeared in the plays Top Girls, Watership Down, Easy Virtue and Touched. After appearing on television, she made her professional stage debut in DNA and The Miracle, two of a trio of one acts directed by Paul Miller at the Royal National Theatre in London; the other one act was Baby Girl. She starred as the main protagonist Amy Dorrit in BBC mini-series Little Dorrit, and was nominated for an RTS Award. She went on to appear in TV movie Going Postal, and is set to