Following the resignation of Albert Pierrepoint in 1956, Allen and Steve Wade jointly became Chief Executioners, but the Homicide Act of 1957 reduced the number of condemned criminals by some 75%, from an average of 15 a year in the early 1950s to 4 a year in the late 1950s. Allen's most controversial case was that of James Hanratty, hanged on 4 April 1962 at Bedford Prison for the A6 murder case, although efforts to clear Hanratty's name continue to this day.
Allen performed one of the simultaneous final executions in Great Britain, when at 8 a.m. on 13 August 1964 Gwynne Owen Evans was hanged at Strangeways Prison in Manchester for the murder of John Alan West, while at the same time his accomplice Peter Anthony Allen was also being hanged at Walton Prison in Liverpool.