McIlwain was born at Liverpool.

He published three issues of a science fiction magazine called The Satellite where he was a co-editor along with J. F. Burke. From 1940 to 1941, he published his own magazine called Gargoyle.

During World War II, he was in the Royal Air Force and served in Northern Africa in 1943.

After the war, he worked in TV engineering, and became involved in editorial work with radio and TV.

During 1952, he sold his first radio play, Spaceways, to the BBC. Due to its popularity, it became a novel as well as a movie.

One of his best known stories, Timeliner, was about a scientist who was experimenting with a time machine, only to be maliciously thrust in the future by a fellow scientist who was having an affair with his wife.It was originally written as a radio play known as The Einstein Highway.

He died at London in 1981.