Deborah Jane Kerr-Trimmer was born on September 30, 1921 in Glasgow. She was the only daughter of Kathleen Rose (née Smale) and Capt. Arthur Charles Kerr-Trimmer, a World War I veteran who lost a leg at the Battle of the Somme and later became a naval architect and civil engineer.
She spent the first three years of her life in the nearby town of Helensburgh, where her parents lived with Deborah's grandparents in a house on West King Street. Kerr had a younger brother, Edmund ("Teddy"), who became a journalist. He was killed in a road rage incident in 2004.