During the Second World War he participated in the campaigns in Greece and Syria as Chief Signals Officer of the I Corps.

While at Caulfield Grammar, Simpson joined the Australian Army Cadets, rising to the rank of sergeant.

[3] Simpson was commissioned as a second lieutenant in the Australian Imperial Force (AIF) on 1 May 1916,[3] and posted to the 3rd Pioneer Battalion,[4] part of the 3rd Division, which was then being raised in Australia.

He was continually under shell-fire, but personally supervised all repairs, by his vigorous and cheerful manner impressing all ranks in the highest degree.

One of the witnesses at their wedding ceremony was a fellow Militia officer, Alan Ramsay, who later married Jean's sister Edna.

[14] In 1937, Simpson banded together with D. E. Robertson and A. E. Moore to create the Allied Master Chemists of Australia Ltd, today better known as Amcal.

When the I Corps was formed in April 1940, Lieutenant General Sir Thomas Blamey was appointed its commander, and Simpson its Chief Signals Officer, with a promotion to the rank of colonel.

He met with his British counterparts and discussed arrangements for the creation of an AIF Signals School in the Middle East.

Both the standard of training of the units and the establishment of the schools intended to remedy the situation were hampered by a serious shortage of equipment.

[18] The 6th Division's list of critical shortfalls—by no means restricted to signals equipment—included 120 telephones and 120 miles (190 km) of electrical cable.

[22] The Battle of Greece presented a major challenge for Simpson's signal units, as rugged terrain, enemy action and frequent troop movements conspired to frustrate their efforts to maintain reliable communications.

The news that Yugoslavia had offered to surrender reached Blamey from a BBC broadcast on 15 April picked up on a receiver built into a kerosene case that Simpson had insisted that he take.

[24] The shortage of signal equipment was an important factor in the delay in committing the I Corps to the Syria–Lebanon campaign until it became clear that General Sir Henry Maitland Wilson could not adequately control operations from his headquarters at the King David Hotel in Jerusalem.

[25] He was mentioned in despatches,[26] and appointed a Commander of the Order of the British Empire for "maintenance of communications under difficult conditions in the Grecian campaign".

[1] He was involved in a serious motor vehicle accident on 13 September 1941 and suffered severe lacerations, a concussion, and a broken collarbone, rib and finger.

These were soon well in hand, but the tactical situation rapidly deteriorated to the extent that the I Corps was ordered to leave Java on 21 February 1942.

He instituted a sweeping reorganisation of the Army, replacing officers with men who had experience in the Middle East.

An important outcome of their first meeting was the creation of the Central Bureau as a combined signals intelligence organisation.

Blamey had an appreciation of signals intelligence from his time as Deputy Commander in Chief in the Middle East, and readily gave his support.

After a slow start, signals intelligence became an important element of the war in the South West Pacific.

[34] In November 1944, he visited the front in the Netherlands, Belgium and France, returning to Australia via the United States and Canada.

[35] To man his signals units, Simpson sought to obtain some 4,000 Australian Women's Army Service (AWAS) personnel.

[1] The large numbers of women serving in Signals units caused friction between Irving and Simpson over what degree of control he exercised over them.

He resigned in October after a dispute with the State Council over its suspension of two members for being communists, which Simpson opposed.