As an actress Thomas worked mainly in Wales, and appeared in such classic films as The Proud Valley (1940) with Paul Robeson, Blue Scar (1949) Valley of Song (1953), and Tiger Bay (1959). In 1943, she appeared as Maria Petrovitch in the Ealing war film Undercover, an account of the guerrilla resistance movement in Yugoslavia during the Second World War.
Thomas mainly performed in Wales as an actress. She starred in films like The Proud Valley (1940) with Paul Robeson, Blue Scar (1949), Valley of Song (1953), and Tiger Bay (1959). In 1943, she played Maria Petrovitch in the Ealing war film Undercover, which chronicled guerrilla resistance in Yugoslavia during World War II.