His acting career began at the age of 16 at the Birmingham Repertory Theatre in 1921. He appeared in many repertory shows until he went to Canada where he worked on a cattle ranch for a year.
He returned to England and performed in many plays in the West End of London, including Bitter Sweet by Noël Coward, The Letter with Gladys Cooper, and Her Cardboard Lover with Tallulah Bankhead. He also appeared in Private Lives on Broadway, taking over the role from his friend Laurence Olivier. From 1932 to 1934, he was the manager of the Shilling Theatre in Fulham, London. He had a small role in the film Reunion (1932).
Newton was put under contract to Alexander Korda who cast him in small roles in the cinema films Fire Over England (1937), Dark Journey (1937), Farewell Again (1937) and The Squeaker (1937). He also had a part as Cassius in the abandoned version of I, Claudius and in 21 Days (shot in 1937, released 1940). Newton was borrowed by 20th Century Fox for The Green Cockatoo (1937). Newton had a good role supporting Charles Laughton in Vessel of Wrath (1938). He had another strong part in Yellow Sands (1939) and had his first film lead in Dead Men are Dangerous (1939). He made another with Laughton, Jamaica Inn (1939), playing the romantic male lead, directed by Alfred Hitchcock. In 1939, he played Horatio to Laurence Olivier's Hamlet at the Old Vic, in a production that included Alec Guinness and Michael Redgrave. Newton kept busy as a , appearing in Poison Pen (1939) and Hell's Cargo (1939).