He was commissioned into the Royal Field Artillery in 1920. During World World War II, Mansergh served with the Royal Artillery in Eritrea, Abyssinia, the Western Desert of Libya, the Middle East, Persia, Iraq, Arakan, Assam and Burma. Having been promoted acting Major-General in 1944, he commanding the 11th (East Africa) Division and the 5th Indian Infantry Division.
Promoted acting lieutenant-general in 1946, Mansergh commanded the 15th Indian Corps and was then appointed Commander-in-Chief of Allied Forces in the Netherlands East Indies.
Mansergh later served as Military Secretary from 1948 to 1949, Commander of British Forces in Hong Kong from 1949 to 1951, Deputy Commander-in-Chief of Allied Forces Northern Europe from 1951 to 1953, Commander-in Chief of Allied Forces Northern Europe from 1953 to 1956 and Commander-in-Chief of the United Kingdom Land Forces from 1956 to 1959. In that capacity he headed a Committee which looked at the Administration of the British Army.
Mansergh also served as Colonel Commandant of the Royal Artillery and the Royal Horse Artillery Regiments.