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After the war, Grantham became Chief of Staff to the Commander-in-Chief, Mediterranean Fleet in 1946. He was appointed Flag Officer Submarines in 1948, Flag Officer, Second in Command of the Mediterranean Fleet in 1950 and Vice Chief of the Naval Staff in 1951. He became Commander-in-Chief, Mediterranean Fleet and NATO Commander Allied Forces Mediterranean in 1954 and then Commander-in-Chief, Portsmouth and Allied Commander-in-Chief, Channel and southern North Sea in 1957. He retired in 1959.