Allen was deputy British High Commissioner in West Germany in 1954. After the end of the Allied occupation regime and the restoration of German sovereignty in 1954, the British High Commission in Bonn became the British Embassy and Allen was Minister (deputy head of mission) 1955–56. He was Ambassador to Greece 1957–61 and Ambassador to Iraq 1961–65. While in Iraq he is reported have been aware of, or even supported, a plot against the Prime Minister Abd al-Karim Qasim which culminated in the February 1963 coup d'état in which Qasim was overthrown and killed.