Born in London, to Scottish parents, Macllwraith trained as an actor at RADA and worked in repertory during the 1950s, performing around the country.
With Tyrone Power in a production of George Bernard Shaw's The Devil's Disciple which ran for a season in London.
By the early 1960s, he had begun to develop a career as a screenwriter with the short film Linda (1960), featuring Carol White, and 8 episodes of the television series, The Human Jungle (1963–64), with Herbert Lom as a Harley Street psychiatrist, among his credits.
[4] His situation comedy Two's Company (1975–79), about an American writer and her English butler, starred Elaine Strich and Donald Sinden.
[3] His other television work includes plays for the Armchair Theatre series broadcast between 1970 and 1972, and episodes of Justice with Margaret Lockwood in a role as a barrister.