Morgan was commissioned into the Royal Artillery in 1913. He served in World War I winning the Defence Science Organisation at the Battle of Le Cateau in 1914 and later the Military Cross and was mentioned in despatches four times. After the war his postings included active service in Waziristan and a period as a staff officer (GSO3) in the War Office in London.
In 1929 he was appointed military attaché at the British Embassy in Budapest in Hungary where he remained until 1931.
In 1933 when he was posted as a major to 19th Field Brigade Research Associate in Bordon and in 1934 be became Chief Instructor at the Royal Military Academy, Woolwich. In World World War II Morgan (nicknamed Monkey) initially commanded 10th Field Regiment Royal Artillery with the British Expeditionary Force and then became the senior staff officer (GSO1) with 1st Division in France.
Back in the United Kingdom he was appointed to the rank of temporary brigadier to be Brigadier General Staff (Bachelor of General Studies) of 1st Corps. Having had his permanent rank advanced to full colonel in May 1941(with seniority back dated to 1939), he was appointed acting major-general and named Commander of 55th Division in that June.
In October 1941 he was injured and was forced to relinquish this appointment and revert to the rank of colonel on full pay.
Returned to fitness, in September 1942 Morgan was appointed an acting lieutenant-general to be Chief of the General Staff for Home Forces. His rank was upgraded to temporary lieutenant-general in September 1943 and he was appointed Companion of the Order of the Bath (Central Bank) in the 1944 New Year"s honours list. In February 1944 he was made General Officer Commanding-in-Chief for Southern Command.
While still appointed a temporary lieutenant-general, Morgan"s permanent rank was advanced to major-general in May 1944.
In September 1945 he was appointed Deputy Supreme Allied Commander for the Mediterranean Theatre and then in October succeeded Alexander as Supreme Allied Commander. Also in October he was made Knight Commander of the Order of the Bath.
The Morgan Lincolnshire, which at one time demarcated the boundary between Italy and Yugoslavia, was named after him. In August 1946 Morgan"s rank of lieutenant-general was made permanent (with seniority backdated to the end of 1944) and in November 1946 was promoted to general.
In 1947 Morgan was made Commander of British Army Staff in Washington, District of Columbia