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.
As an actress Thomas primarily acted in Wales, and appeared in classic films such as The Proud Valley (1940) with Paul Robeson, Blue Scar (1949), Valley of Song (1953), and Tiger Bay (1959). In 1943, she portrayed Maria Petrovitch in the Ealing war film Undercover, depicting the guerrilla resistance movement in Yugoslavia during World War II.