Unfit for military service because of his polio, he joined MI5 in November 1939, two months after the start of World War II, upon Ball's recommendation.
Chapman Pincher and
Nigel West both suggest that it was Ball's influence that enabled Mitchell to be accepted into that organisation; Ball himself had been in MI5 until 1927, and was later appointed to the highly secret home defence security executive.
World War II Mitchell spent most of the war at
Blenheim Palace, in
Oxfordshire, to which most of MI5 was evacuated in 1940 due to bombing threats in London. Mitchell was a member of F Division, which was for monitoring subversion, and which was headed by
Roger Hollis, who had joined just before Mitchell. His subsection's role was to maintain surveillance on suspected
Nazi sympathizers and right-wing nationalist organisations such as the
British Union of Fascists as well as German and Austrian political bodies. Mitchell assisted Francis Aiken-Sneath in investigating Sir
Oswald Mosley's activities and in organising the case for his wartime detention.
After World War II After the war Mitchell became director of F Division, and in 1953 moved to become head of D Division (counter-espionage). In May 1951 diplomats
Guy Burgess and
Donald Maclean had defected to Moscow, and Mitchell led the MI5 team investigating what Soviet penetration there might have been in Britain's intelligence services. He had a major role in introducing "
positive vetting" for civil servants with access to highly classified information. Mitchell was the principal author of the 1955 White Paper concerning the disappearance of Burgess and Maclean, under the close supervision of Hollis. This document made no mention of
Kim Philby, and led to
Harold Macmillan exonerating Philby after he had been named in the
House of Commons.
Chapman Pincher, an investigative journalist specialising in the intelligence services, wrote of the paper "it was strewn with statements now proven to be false – as they were known to be inside MI5 at the time". After the war Mitchell was responsible for the surveillance of fascist
John Beckett. Beckett's son
Francis studied the MI5 records of his family's case, including Mitchell's memoranda, when they were released in 2016. While not defending or agreeing with his father or supporting his detention, Francis Beckett said of Mitchell "the satisfaction in the covert control he exerted over other people shines through his flat prose", and that the memoranda show signs of reluctance to later give up the arbitrary power over the freedom of others that had been granted in 1940. In 1956 Roger Hollis became director general of MI5, and appointed Mitchell to be his deputy.
Suspicions of Soviet penetration and the "Fluency" investigation MI5's performance in counterespionage had been notably unsuccessful in the 1950s. Its own investigations had led to only one spy being caught and no Soviet defectors having been recruited. This was in stark contrast to its highly effective performance during WWII. This led to suspicions within the
Secret Intelligence Service that MI5 had become infiltrated by a Soviet "mole". Suspicion fell on both Hollis and Mitchell although any evidence was highly circumstantial. A few years later, it seemed highly likely that Kim Philby, in the SIS at the time of the Burgess–Maclean defections, had been tipped off that he was about to be confronted with conclusive evidence of his treachery, and this led to his decampment from
Beirut to Moscow in 1963. A complicating factor was that Sir
Dick White had been director general of MI5 between 1953 and 1956 before he became director general of the SIS in 1956. With White's authority,
Arthur S. Martin approached Hollis in early 1963, shortly after Philby defected, to get permission to investigate the possible tip-off of Philby, without naming any suspects. Hollis agreed to this if White also approved. Martin then told White that Mitchell and Hollis himself were the main suspects. White contacted Hollis but only to mention Mitchell as a suspect, and the molehunt, codenamed "Fluency", was officially started. In September 1963, Mitchell unexpectedly took early retirement for health reasons at age 58, after 24 years service. He had announced that he would do so before he came under suspicion. But he was later interrogated in 1968, and seemingly was able to answer the charges successfully. The main suspicion then fell on Hollis and, although the matter has never been completely resolved,
Christopher Andrew in his
Authorized History of MI5 comes to a firm conclusion that neither were traitors. Pincher, conversely, believes that Hollis was the most likely culprit, if there was one. ==Public discussion about investigations into Hollis and Mitchell==