Barton began acting in the theatre at the age of 12. Because she was small for her age, during World War 2 she became known for playing children and young people. At 17 she appeared in "Pink String and Sealing Wax" at the Duke of York's Theatre and remembered performing while doodlebugs flew overhead. While on the West End stage she was spotted by David Lean and Noël Coward and cast for the part of Beryl in "Brief Encounter". Her scenes were shot at Denham Film Studios and for other scenes she was invited by Lean to watch from behind the camera.