Along with his wife Lucy Oldham (codename MADAM) he spied for the Soviet Union between 1929 and his death in 1933, in return for money.

He had no apparent ideological interest in helping the Soviet Union (unlike the more famous Cambridge Five), but was driven by the large amounts of money paid to him to betray his country.

By 1933, the pressures of his activities had led to his sacking from the Foreign Office, alcoholism, domestic violence and ultimately suicide.

Despite hints to there being a spy within the Foreign Office by Soviet defectors Grigory Besedovsky (in October 1929) and Georges Agabekov (in June 1930), Oldham's espionage was only partly suspected by his employers during the last months of Oldham's life, when MI5 began their investigation and surveillance.

He was found dead on 29 September 1933 at 31 Pembroke Gardens, Kensington, London, with his head in a gas oven, and although ostensibly a suicide, it is just as likely that he was killed by the Soviets.