Born in Banbridge, Northern Ireland, McWilliam studied painting at the Slade School of Fine Art in London from 1928 to 1931 and then worked in Paris for a year. He began making sculptures after a visit to Brancusi's studio in 1933. McWilliam joined the English surrealist group in 1938. By that time he was producing sculptures in which he fragmented and distorted the human form to produce objects which could be both sinister and darkly humorous in appearance. After the Second World War McWilliam's work became more rugged, and often naturalistic, in style.