Born in London into a theatrical family, Cross started off by writing children's plays in the 1950s. He achieved instant success with his first 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.
Cross' second play, Strip the Willow, was to make a star out of his future wife, Dame Maggie Smith, even though the play was staged only in the provinces, never receiving a London production. In 1962, he translated Marc Camoletti's classic farceBoeing Boeing, which went on to have a lengthy and highly lucrative run in the West End. In 1964, he directed the play in Sydney. 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.