Leonard Monk Isitt was born on 27 July 1891 in Christchurch, New Zealand, the son of the Methodist minister, member of parliament and prohibitionist Leonard Monk Isitt and Agnes Martha Caverhill. Leonard Monk Isitt junior was educated at Mostyn House, Cheshire, England and Christchurch Boys' High School. He had one brother, Willard Whitmore Isitt (1894–1916) who was a Rifleman in the New Zealand Rifle Brigade in World War I and was killed in France on 31 October 1916.