During World War II, he was a Spitfire production test pilot and flying instructor. This led to him being offered the role of test pilot Philip Peel in Breaking the Sound Barrier (1952). However, he had no desire to fly professionally and never flew an aircraft in real life after 1945.
He and Barbara Murray had three daughters.
His Argentinian father became a naturalized British subject and his mother was English.
His wife Alison McMurdo was a 5' 6 1/2" stage actress who appeared in such plays as "The Brides of March".
John Justin learnt to fly in Argentina at the age of 12, but was refused permission to go solo by the authorities due to his age.