Cawthorn married Mary Wyman Varley in 1927 in London and they had one son, Michael John Douglas Cawthorn, who was born 10 March 1930. His son Michael would later attend the Royal Military Academy Sandhurst, and was commissioned into the Argyll and Sutherland Highlanders on 16 December 1949. However, he was killed serving in Korea on 4 April 1951 with the 1st Battalion. Cawthorn was appointed a company commander in the 4th Battalion, 16th Punjab Regiment on 1 May 1930 and served on the North West Frontier that year.