Hollis moved to China and worked as a reporter on the Shanghai Post and as a representative of the British-American Tobacco Company. After developing tuberculosis in 1939 Hollis returned to England.
On 5th September 1945, Igor Gouzenko, a cipher clerk in the Russian Legation, defected to the West claiming he had evidence of an Soviet spy ring based in Britain. The case was passed on to Kim Philby, head of Section IX (Soviet Affairs) of MI6. He suggested that Gouzenko should be interviewed by Hollis.
Gouzenko provided evidence that led to the arrest of 22 local agents and 15 Soviet spies in Canada. Information from Gouzenko resulted in the arrest and conviction of Klaus Fuchs and Allan Nunn May. Gouzenko also claimed that there was a Soviet agent inside MI5. However, he was later to argue that Hollis showed little interest in this evidence. "The mistake in my opinion in dealing with this matter was that the task of finding the agent was given to MI5 itself. The results even beforehand could be expected to be nil."
Hollis replaced Sir Dick White as Director General of MI5 in 1956. When Kim Philby escaped to the Soviet Union in 1962 rumours began to circulate that Hollis had tipped him off that he was about to be arrested. Hollis was also criticised for not telling John Profumo that he was involved with Stephen Ward, a suspected Russian spy.
In 1963 Michael Straight was offered the post of the chairmanship of the Advisory Council on the Arts by President John F. Kennedy. Aware that he would be vetted - and his background investigated - he approached Arthur Schlesinger, one of Kennedy's advisers, and told him that Anthony Blunt had recruited him as a spy while an undergraduate at Trinity College. Schlesinger suggested that he told his story to the FBI.
Straight's information was passed on to MI5 and Arthur Martin, the intelligence agency's principal molehunter, went to America to interview him. Straight confirmed the story, and agreed to testify in a British court if necessary. Christopher Andrew, the author of The Defence of the Realm: The Authorized History of MI5 (2009) has argued that Straight's information was "the decisive breakthrough in MI5's investigation of Anthony Blunt".
Peter Wright, who took part in the meetings about Anthony Blunt case, argues in his book, Spycatcher (1987) that Roger Hollis decided to give Blunt immunity from prosecution because of his hostility towards the Labour Party and the damage it would do to the Conservative Party: "Hollis and many of his senior staff were acutely aware of the damage any public revelation of Blunt's activities might do themselves, to MI5, and to the incumbent Conservative Government. Harold Macmillan had finally resigned after a succession of security scandals, culminating in the Profumo affair. Hollis made little secret of his hostility to the Labour Party, then riding high in public opinion, and realized only too well that a scandal on the scale that would be provoked by Blunt's prosecution would surely bring the tottering Government down."
Anthony Blunt was interviewed by Arthur Martin at the Courtauld Institute on 23rd April 1964. Martin later wrote that when he mentioned Straight's name he "noticed that by this time Blunt's right cheek was twitching a good deal". Martin offered Blunt "an absolute assurance that no action would be taken against him if he now told the truth". Martin recalled: "He went out of the room, got himself a drink, came back and stood at the tall window looking out on Portman Square. I gave him several minutes of silence and then appealed to him to get it off his chest. He came back to his chair and confessed." He admitted being a Soviet agent and named twelve other associates as spies including Straight, John Cairncross, Leo Long, Peter Ashby and Brian Symon.
Arthur Martin was disappointed when it was discovered that Roger Hollis and the British government had decided not to put Anthony Blunt on trial. Martin once again began to argue that there was still a Soviet spy working at the centre of MI5. Hollis thought Martin's suggestion was highly damaging to the organization and ordered Martin to be suspended from duty.
After his suspension had ended, Martin, along with Martin Furnival Jones and Peter Wright, went to see Dick White, head of MI6. They told White that they were convinced that either Hollis or his deputy, Graham Mitchell, were Soviet agents. White contacted Hollis and it was agreed that Mitchell should be kept under constant surveillance.
In 1964 Hollis ordered the investigation into Mitchell should be brought to an end. Arthur Martin protested by accusing Hollis of protecting Mitchell. Hollis was furious and took his revenge by replacing Martin with Ronald Symonds as head of DI (Investigations). Soon afterwards Martin was sacked from MI5.
Peter Wright now became convinced the real Soviet mole was Hollis. After carrying out further research into Hollis he discovered that while at university he became a close friend of Claude Cockburn, a suspected KGB agent. Although Hollis knew that MI5 had been investigating Cockburn for many years he had never revealed details of this relationship. Wright also found out that Hollis had been in contact with Agnes Smedley, another suspected Soviet agent, while he was in China.
Wright was unable to convince any other senior member of the MI5 that Hollis was a spy. Hollis retired in 1965 and was replaced by Martin Furnival Jones. Sir Roger Hollis died in October 1973.
In his retirement Peter Wright wrote an account of his work at MI5. Despite attempts by Margaret Thatcher and her government to suppress the publication and distribution of the book, Spycatcher, was published in 1987. In the book Wright claimed that Hollis had been a Soviet double-agent and had been the fifth man in the spy ring that included Kim Philby, Guy Burgess, Donald Maclean and Anthony Blunt.