Born in Dorset in the West Country of England and growing up in Cornwall near Lands End, his exaggeration of his West Country accent is credited with popularizing the stereotypical "pirate voice".
Newton's acting career began at the age of 16 at the Birmingham Repertory Theatre in 1921. Later, he performed in many plays in the West End of London, including Bitter Sweet, The Letter with Gladys Cooper, and Cardboard Lover. He also appeared in Private Lives on Broadway, taking over a role from his friend, Laurence Olivier.
Transitioning to film work, Newton began with small roles in Fire Over England, Dark Journey, Farewell Again, and The Squeaker before becoming more prominent in films, like Vessel of Wrath, Dead Men Are Dangerous, Jamaica Inn, Gaslight, Busman's Honeymoon, Bulldog Sees It Through, Major Barbara, This Happy Breed, Henry V, Oliver Twist, Obsession, Les Misérables, Androcles and the Lion, Blackbeard the Pirate, The Desert Rats, The Beachcomber, and Around the World in 80 Days. He was considered to voice Captain Hook in Peter Pan. He was also considered for the role of Friar Tuck in The Story of Robin Hood and His Merrie Men and Captain Nemo in 20,000 Leagues Under the Sea.
Following his success of Treasure Island, Newton was approached by producer Joseph Kaufman to reprise his role for his independently produced sequel, Long John Silver. The film was poorly received, but it still spawned a 26-episode 1955 TV series, The Adventures of Long John Silver.
Newton was married four times and died on March 25, 1956 following a heart attack. His body was cremated and his ashes were later scattered into the English Channel in Mount's Bay, near Lamorna in Cornwall. He was posthumously honored as a Disney Legend in 2002.