During the Second World War, he served as Chief of Staff to Field Marshal Harold Alexander, and later succeeded him as Supreme Allied Commander in the Mediterranean Theatre of Operations.

[2] He attended the Staff College, Camberley from 1925 to 1926, where his fellow students included Ronald Scobie, Frank Messervy, Raymond Briggs, Eric Harrison, Henry Willcox, Francis Tuker, John Swayne and Ralph Deedes.

[2][1] In the Second World War, Morgan (nicknamed "Monkey")[9] initially commanded the 10th Field Regiment, Royal Artillery with the British Expeditionary Force, before succeeding Richard McCreery as the General Staff Officer Grade 1 with the 1st Infantry Division in France.

[16] In March 1945,[17] he became Chief of Staff to the Supreme Allied Commander in the Mediterranean Theatre, Field Marshal Sir Harold Alexander.

[7] In this capacity Morgan was offered access to the atomic bomb by General Dwight D. Eisenhower as an incentive to persuade Britain to give up its own programme.

[22] His knighthood was advanced to Knight Grand Cross of the Order of the Bath in the 1949 New Year Honours,[23] and he retired from the British Army in June 1950.