George Leonard Marshall enlisted in August 1915 as a trooper with the Canterbury Mounted Rifles, Eighth Reinforcements. In Egypt by the end of the year, Marshall served in the Sinai before contracting scarlet fever in March 1916. Shortly after, he transferred to the NZ Field Artillery as a gunner. In 1917-8 he transferred again, this time to the First Canterbury Infantry Regiment to join his younger brother Hector. Both brothers were wounded at Messines on 7 June 1917. Though wounded himself, George Marshall carried his brother one mile across the battle field to safety. Hector was subsequently invalided back to New Zealand while George recuperated in England, rejoining his regiment in November of that year.

During WW2 Marshall joined the Home Guard where he trained machine gunners.