Roberts was born in Wivelsfield in June 1907, the son of Percival Arthur Garfield Roberts and his wife Maud (née Green). At the time of the 1911 census, the family was living in Penzance, where his father worked for the Western Union Cable Company. Eric eventually settled with his family in Epsom.
He became a bank clerk in the Euston Road branch of the Westminster Bank in London. He spoke fluent Spanish and some German following holidays there in 1932 and 1934. He married Alice Lilian Audrey Sprague (born 1900 Droylesden, Lancashire), the daughter of William Sprague, a railway civil engineer, and his wife Margaret, in spring 1934.
Roberts was recruited at the age of 17 (in 1925) by the MI5 spymaster Maxwell Knight. This was during his employment as a bank clerk for Westminster Bank After Roberts' employer was asked to release him for war service, one of his bosses wrote back, expressing surprise as to why they would choose someone so unremarkable for important work: "what are the particular and especial qualifications of Mr Roberts - which we have not been able to perceive - for some particular work of national military importance which would take him away from his normal military call-up in October?"
By May 1940, he was posing as a German Gestapo agent named "Jack King", a member of the Einsatzgruppe London, to obtain information about Nazi sympathisers in the UK. There had been speculation that King was John Bingham, until the release of files by MI5 in October 2014. Documents in the UK National Archives have now shown that Roberts ran a hugely dangerous and very successful deception.
From 1942, as Jack King, he was in direct contact with six men and women who believed he was working for the Germans; they gave him information on "scores and probably hundreds" of Nazi sympathisers in the UK. Hardened spies were "astonished" by the treachery he unearthed. Originally, his mission was to infiltrate Siemens Schuckert (GB) Ltd, the suspect British arm of the German company, until he met a "crafty and dangerous woman" named Marita Perigoe. Of mixed Swedish and German origin, she was married to a member of Oswald Mosley's British Union of Fascists, but saw them as "insufficiently extreme". MI5 reported that "She was found to be so violently anti-British and so anxious to do anything in her power to help the enemy that it was felt that special attention should be paid to her." One Nazi sympathiser, Hilda Leech, passed on reports about secret research being undertaken to develop a jet aircraft. An astrologer, Edgar Whitehead, gave details about secret trials on a new amphibious tank. The naturalised British citizen Hans Kohout uncovered information about secret British tactics to evade air defences and passed this on to Jack King.
The fifth column fascists' hatred for Britain, driven by anti-Semitism and the propaganda of Mosley’s group, was so strong that they "applauded" women and children being killed by German bombs, according to Roberts' reports. Thus, Roberts was able to prevent the passing on of sensitive information to Germany. His detailed and extensive groundwork produced a list of 500 names of people of interest to MI5.
In 1947 Roberts was seconded to the Secret Intelligence Service (MI6) to work in Vienna, posing as a British civil servant and passing information to the Soviet agent Jellinek. This low grade, accurate but harmless information passed to enemy spies he described as "chicken feed", a term also used by the spy writer John le Carré. At the time Vienna was a very dangerous place full of spies from all sides as portrayed in the film, The Third Man. Roberts wrote about some of the dangers he faced everyday in his work in Austria. In one incident he had waited for an arranged meeting at a cemetery in the city and narrowly avoided being picked up by a Russian ZiL car.
In contrast to the brilliance of Roberts work before and during the War, he had effectively been moved to a much less demanding and stimulating desk job which caused him to become depressed, and his work at this point was viewed much less favourably by his superiors in MI5 and MI6.
On returning to London, Roberts asked his friend Guy Liddell, deputy director of MI5, about a double agent that had been uncovered in Vienna. Roberts told him that he thought it likely that there would be a mole in MI5 and that anyone with the right background, and a member of the correct clubs, would be above suspicion. Such a spy would likely be motivated by ideology rather than financial gain he suggested. It was following this conversation that Roberts felt that he himself had come under suspicion.
After leaving the security service, the Cambridge Five, a Soviet group of double agents including Guy Burgess, Donald Maclean, and Kim Philby was uncovered by MI5. This led to Roberts being visited in Canada by Barry Russell Jones from MI5 to find out about any suspicions he might have about his former colleagues. Roberts passed them the name "Tony", whom they had already identified as one of the five, Anthony Blunt, but he again felt himself to be under suspicion and was reported to have been traumatised by the visit. From Roberts' papers, it appears that he had doubts about Blunt from a very early stage and tried to raise his concerns in 1941.
In a 14-page letter written in 1969 to Harry Lee of MI5, Roberts wrote of his anxiety and frustration about his time at the service, believing he was being followed and spied on by agents in London during that time. He spoke of not having felt accepted by colleagues at MI5 since he was from grammar school and did not have the same public school and Oxford or Cambridge background. Harry Lee replied, to apologise for the visit in Canada, and tried to reassure Roberts that he had not been under suspicion or followed.
Roberts retired in 1956, at the age of 49, and lived in Nettlestone, Isle of Wight, before emigrating to Canada, eventually settling in Ganges, Saltspring Island, British Columbia, where he died on 17 or 18 December 1972. He left a widow Audrey, sons Maxwell (born 17 January 1936) and Peter, daughter Christa (McDonald), three grandchildren and two sisters. He was a freeman of the City of London and author of the book Salt Spring Saga.