Simpson was commissioned as a second lieutenant in the Australian Imperial Force (AIF) on 1 May 1916, and posted to the 3rd Pioneer Battalion, part of the 3rd Division, which was then being raised in Australia.
He embarked from Port Melbourne on the transport HMAT Wandilla on  6 June 1916, arriving in England on 26 July 1916.
The 3rd Division trained on the Salisbury Plain in England, where he was promoted to lieutenant on 13 October 1916.
He was transferred to the 3rd Division Signal Company on 16 November 1916.
Soon after, the 3rd Division moved to the Western Front, moving into the line near Armentières.
Simpson was mentioned in despatches on 4 January 1917.


The 3rd Division carried out its first offensive at the Battle of Messines in June 1917.
For his part in the battle, Simpson was awarded the Military Cross.
His citation read: 
For conspicuous gallantry and devotion to duty.
He organised the Brigade Signal Service so thoroughly that communication was maintained with every unit throughout.
He was continually under shell-fire, but personally supervised all repairs, by his vigorous and cheerful manner impressing all ranks in the highest degree.

Simpson participated in the Battle of Passchendale where he was gassed and wounded.
He was evacuated to England on 22 October 1917.
While in hospital he applied for nine months' leave to return to Australia and complete his pharmaceutical studies.
He had passed the Intermediate Examination before leaving Australia but not the Final Examination.
This was granted, and he embarked for home on the transport HMAT Persic on 21 December 1917.
He passed the Final Examination, and was registered as a pharmacist on 10 July 1918.
He never returned to the front, and his AIF appointment was terminated on 9 August 1918.
Simpson was commissioned as a second lieutenant in the Australian Imperial Force (AIF) on 1 May 1916, and posted to the 3rd Pioneer Battalion, part of the 3rd Division, which was then being raised in Australia. He embarked from Port Melbourne on the transport HMAT Wandilla on  6 June 1916, arriving in England on 26 July 1916. The 3rd Division trained on the Salisbury Plain in England, where he was promoted to lieutenant on 13 October 1916. He was transferred to the 3rd Division Signal Company on 16 November 1916. Soon after, the 3rd Division moved to the Western Front, moving into the line near Armentières. Simpson was mentioned in despatches on 4 January 1917.

The 3rd Division carried out its first offensive at the Battle of Messines in June 1917. For his part in the battle, Simpson was awarded the Military Cross. His citation read: 
For conspicuous gallantry and devotion to duty. He organised the Brigade Signal Service so thoroughly that communication was maintained with every unit throughout. He was continually under shell-fire, but personally supervised all repairs, by his vigorous and cheerful manner impressing all ranks in the highest degree.
Simpson participated in the Battle of Passchendale where he was gassed and wounded. He was evacuated to England on 22 October 1917. While in hospital he applied for nine months' leave to return to Australia and complete his pharmaceutical studies. He had passed the Intermediate Examination before leaving Australia but not the Final Examination. This was granted, and he embarked for home on the transport HMAT Persic on 21 December 1917. He passed the Final Examination, and was registered as a pharmacist on 10 July 1918. He never returned to the front, and his AIF appointment was terminated on 9 August 1918.
Colin Hall Simpson