Mcwilliam was born in Northern Ireland and studied at the Slade School of Art, London. His first solo exhibition was at the London Gallery in 1939. In recent years McWilliam lived and worked in the Holland Park area. He was a good friend of the painter William Scott, and McWilliam made nude portrait busts of both William and his wife Mary. Mary Scott met her husband when she was a student at the Royal Academy Schools, where she studied sculpture. They both sat to McWilliam at the beginning of 1956, and he completed the portraits of them both in a week each.