Jill Angela Henriette Balcon was an English film and radio actress. She made her film debut in Nicholas Nickleby, though she was best known for her stage, television and radio work.
Balcon was born in Westminster, London, the daughter of Aileen Freda Leatherman and Michael Balcon. Her family was Jewish, and originated in Lithuania on her father's side and Poland on her mother's. Balcon attended Roedean.
In 1951, Balcon married Anglo-Irish poet Cecil Day-Lewis, who was 20 years her senior and was married when they began their relationship. He was dividing his time between his mistress Rosamond Lehmann in Oxfordshire, his wife, Mary Day-Lewis, and their two teenage sons, in Devon.
Cecil Day-Lewis broke with both his wife and his mistress to be with Balcon for the remainder of his life. Balcon was cut off by her father when she married Day-Lewis in 1951. Cecil Day-Lewis and Jill Balcon had two children: actor Daniel Day-Lewis and documentary filmmaker/television chef Tamasin Day-Lewis.