He was born in London's East End, and attended Cambridge University where he joined the University's Socialist Society and met his wife Ann, a Communist Party member. MI5 first became interested in Mankowitz in 1944, when he and his wife were mentioned in a letter from suspected communist David Holbrook, prompting the agency to ask Newcastle police to investigate them.
Holbrook had written that the couple were 'avoiding National Service and doing themselves well' earning 6 pounds a week lecturing for the left-wing Workers' Educational Association. In their report to MI5, the Newcastle police said Mankowitz 'is known to frequently discuss the theories of Marxism with his friends whilst in lodgings'.
Mankowitz, who then enlisted with the Territorial Army, was described by his commanding officer as a 'highly strung individual of nervous temperament' who was awaiting an interview with a psychiatrist.
But he doubted he was a subversive influence, as he did not possess the personality or strength of character to pass them on to his fellow soldiers. "There is no evidence that he has attempted to air these views whilst with this unit," the BBC quoted the officer as having written.