Born in Northamptonshire, England, Guillory is the daughter of the American folk guitarist Isaac Guillory and his first wife, Tina, whom he married in 1973. Isaac Guillory was of Jewish heritage and born at the Guantanamo naval base in Cuba, the son of Easton Joseph Guillory, an American sailor, and Victoria Ojalvo, who was from a Cuban-Turkish family.

Her parents moved to Fulham, London when Guillory was two, then later to Norfolk when she was eleven. At the age of ten, she went to stay with cousins in Mexico to learn Spanish. Raised in bohemian households, she has a step brother named Jace, whom her father had adopted. Her parents divorced in 1990 when she was fourteen, and in 1993 her father married Vickie McMillan, which resulted in a much younger half-sister and half-brother, Eleanor and Jacob. Guillory attended Gresham's School in Holt, Norfolk, where she took part in numerous school productions. She was later expelled from the school for stealing.

Guillory has bee