A 1963 letter included in Corporal Marshall's Army Personnel File, signed by the Army Secretary, notes that as individual citations do not accompany these decorations, it isn't possible to "identify the particular act or acts of gallantry for which ... [he] received this distinction. On 30 September 1918 his battalion was hotly engaged with the enemy near Crevecours (sic) at the crossing of the Scheldt Canal and it is presumed he distinguished himself in this action." However, the relevant entry in Honours and Awards to the New Zealand Expeditionary Force in the Great War 1914-1918 by Wayne McDonald suggests the award was made in recognition of Marshall's earlier actions in the field near Bapaume on 6-7 September 1918.

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.