He was born into a theatrical family, and started off by writing children's plays in the 1950s. He achieved instant success with his debut play One More River, which dealt with a mutiny in which a crew puts its first officer on trial for manslaughter. The play made its debut in 1959, starring Michael Caine and directed by Laurence Olivier.
Cross' second play Strip the Willow was to make a star out of his future wife Dame Maggie Smith. In 1962, he translated Marc Camoletti's classic farce Boeing Boeing, which went on to have a lengthy and highly lucrative run in the West End. He even directed the play in Sydney in 1964. Another of his successes was Half a Sixpence, a musical comedy based on the H.G. Wells novel Kipps. This opened in 1963, and like his first play, ran in London for more than a year.
Cross later became well known for his screenplays, notably Jason and the Argonauts, The Long Ships, Genghis Khan, and Clash of the Titans. He also adapted Half a Sixpence for the screen. He also worked uncredited on the script for Lawrence of Arabia, although it's doubtful any of his material made it to the final cut.