Alexander enlisted on 28 July 1915. He was the twenty five year old son of Ross Hill School Headmaster, A.K. Gilchrist and worked as a Linotype operator. He was a keen cricket player and in a local match in January 1915 had taken 5 wickets for 25 runs. He was also a member of the Inverell Rifle Club. Prior to his departure for war service Alexander was given a surprise farewell at the home of his parents. The Inspector of Schools presented him with a gift of a money belt and others gave him a sheepskin vest.
As an Acting Corporal with the 20th Battalion 9th Reinforcements, Alexander sailed from Sydney on the HMAT A54 Runic on 20 January 1916 and served in France. By the end of the year he had been promoted to Sergeant and during 1918 was moved to 2nd Division Headquarters. He was awarded the Meritorious Service Medal in September 1918 for his '... very high standard of devotion to duty and his reliability and zeal ...'
Sergeant Gilchrist returned to Australia in May 1919 and, due to the termination of enlistment with the AIF, was discharged. He is among those listed on the Inverell Honor Roll.