[2] After attending the Guildhall School of Music and Drama in London, McCallum's first major appearance on stage was alongside Sam Wanamaker in The Rainmaker in the mid-1950s.

[3] He appeared in British TV series in the 1950s, 1960s and early 1970s, including The Saint, Department S and UFO, and in films such as The Siege of Pinchgut (1959) (playing the brother of Aldo Ray[4]) and Dr. Terror's House of Horrors (1965).

[5] He provided the voices of Dr Ray Pierce in the film Thunderbirds Are Go (1966) and an airport controller in the TV series Captain Scarlet and the Mysterons (1967).

[6] He played lead character Angelo James in the BBC TV series Vendetta (1966–1968).

McCallum lived at George's Farm, Crookham Common, near Thatcham, until his sudden death on 26 April 1976.