Malley joined the Australian Imperial Force (AIF) on 12 October 1915.
He departed Melbourne for Egypt as a gunner with reinforcements of the 1st Field Artillery Brigade aboard HMAT Wandilla on 9 November.
In March 1916, he was posted to the Western Front in France, and joined his unit in May. The 1st Brigade took part in the Battles of Pozières and Mouquet Farm in July and August 1916.
Malley transferred to the Australian Flying Corps (AFC) as a mechanic in April 1917, before undertaking flying instruction at the Oxford University air school.
He was commissioned a second lieutenant on 9 October and assigned to No.
 4 Squadron AFC, then based in Birmingham.

Equipped with Sopwith Camel fighters, No.
 4 Squadron deployed to Bruay, France, in December 1917, and commenced operations on 9 January 1918.
That same day Malley, nicknamed "Garnie" (or "George", by No.
 4 Squadron's leading ace, Harry Cobby), was promoted lieutenant.
On 16 March, he achieved his first aerial victory, sending a fighter belonging to Manfred von Richthofen's Red Circus out of control above Annoeullin, near Douai.
Early reports identified the German plane as an Albatros, but later sources record it as a Pfalz D.III.
Malley claimed two Albatroses on 23 March 1918, during an attack on German positions in Vaulx-Vraucourt, near Bapaume.
Three days later he was promoted captain and appointed a flight commander.

The official history of Australia in the war credits Malley with the destruction of a Pfalz over Wytschaete on 10 May, though it does not appear in other accounts of his final tally.
Four days later, he and Lieutenant Roy King each claimed a German two-seater spotting for artillery between Ypres and Bailleul.
On 30 May, Malley and Cobby led their flights on a bombing mission in the Lys region, after which they each destroyed a German observation balloon over Estaires.
Malley's final victory, over the Lys on 1 June, was a Pfalz D.III.
His official tally was six German aircraft destroyed—four fighters, an observation balloon, and an unidentified observation plane—and he was wounded in action twice, by a bullet through the leg in March, and by shrapnel from anti-aircraft fire in May. He was awarded the Military Cross on 22 June.
The citation was promulgated in The London Gazette:
Lt. (
T./Capt.)
Garnet Francis Malley, Aust.
F.C., attd.
R.F.C.
For conspicuous gallantry and devotion to duty.
When on offensive and low-flying patrol he attacked one of two hostile scouts, which eventually turned over and fell out of control, being seen to crash by another pilot.
Later, a general engagement ensued with four enemy scouts, one of which he attacked, with the result that it fell completely out of control and crashed.
Prior to this occasion he had also shot down out of control another hostile machine.
His courage and able leadership have resulted in his patrol carrying out excellent work under the most adverse conditions.

Malley took temporary command of No.
 4 Squadron at the end of June 1918, overseeing its move from Clairmarais North to a new airfield at Reclinghem.
In August, he was posted to No.
 5 (Training) Squadron AFC at Minchinhampton, England.
The squadron was part of the 1st Training Wing, led by Lieutenant Colonel Oswald Watt.
Malley's rotation to home establishment was in accordance with Royal Air Force policy requiring pilots to be rested and serve as instructors after nine to twelve months in combat.
Known for flying a white Camel trainer, he received the Air Force Cross for his instructional work; the award was promulgated on 3 June 1919.
Malley joined the Australian Imperial Force (AIF) on 12 October 1915. He departed Melbourne for Egypt as a gunner with reinforcements of the 1st Field Artillery Brigade aboard HMAT Wandilla on 9 November. In March 1916, he was posted to the Western Front in France, and joined his unit in May. The 1st Brigade took part in the Battles of Pozières and Mouquet Farm in July and August 1916. Malley transferred to the Australian Flying Corps (AFC) as a mechanic in April 1917, before undertaking flying instruction at the Oxford University air school. He was commissioned a second lieutenant on 9 October and assigned to No. 4 Squadron AFC, then based in Birmingham.
Equipped with Sopwith Camel fighters, No. 4 Squadron deployed to Bruay, France, in December 1917, and commenced operations on 9 January 1918. That same day Malley, nicknamed "Garnie" (or "George", by No. 4 Squadron's leading ace, Harry Cobby), was promoted lieutenant. On 16 March, he achieved his first aerial victory, sending a fighter belonging to Manfred von Richthofen's Red Circus out of control above Annoeullin, near Douai. Early reports identified the German plane as an Albatros, but later sources record it as a Pfalz D.III. Malley claimed two Albatroses on 23 March 1918, during an attack on German positions in Vaulx-Vraucourt, near Bapaume. Three days later he was promoted captain and appointed a flight commander.
The official history of Australia in the war credits Malley with the destruction of a Pfalz over Wytschaete on 10 May, though it does not appear in other accounts of his final tally. Four days later, he and Lieutenant Roy King each claimed a German two-seater spotting for artillery between Ypres and Bailleul. On 30 May, Malley and Cobby led their flights on a bombing mission in the Lys region, after which they each destroyed a German observation balloon over Estaires. Malley's final victory, over the Lys on 1 June, was a Pfalz D.III. His official tally was six German aircraft destroyed—four fighters, an observation balloon, and an unidentified observation plane—and he was wounded in action twice, by a bullet through the leg in March, and by shrapnel from anti-aircraft fire in May. He was awarded the Military Cross on 22 June. The citation was promulgated in The London Gazette:
Lt. (T./Capt.) Garnet Francis Malley, Aust. F.C., attd. R.F.C.
For conspicuous gallantry and devotion to duty. When on offensive and low-flying patrol he attacked one of two hostile scouts, which eventually turned over and fell out of control, being seen to crash by another pilot. Later, a general engagement ensued with four enemy scouts, one of which he attacked, with the result that it fell completely out of control and crashed. Prior to this occasion he had also shot down out of control another hostile machine. His courage and able leadership have resulted in his patrol carrying out excellent work under the most adverse conditions.
Malley took temporary command of No. 4 Squadron at the end of June 1918, overseeing its move from Clairmarais North to a new airfield at Reclinghem. In August, he was posted to No. 5 (Training) Squadron AFC at Minchinhampton, England. The squadron was part of the 1st Training Wing, led by Lieutenant Colonel Oswald Watt. Malley's rotation to home establishment was in accordance with Royal Air Force policy requiring pilots to be rested and serve as instructors after nine to twelve months in combat. Known for flying a white Camel trainer, he received the Air Force Cross for his instructional work; the award was promulgated on 3 June 1919.
Garnet Malley