On 29 July 1915, Baker enlisted in the Australian Imperial Force for service during the First World War.
Allocated as a reinforcement to the 6th Field Artillery Brigade with the rank of gunner, he embarked from Melbourne aboard HMAT Persic on 22 November, bound for Egypt.
On arrival, Baker was posted to the 16th Battery before moving to France for service on the Western Front.
Disembarking in France on 1 July 1916, Baker took part in the Somme offensive.

On 11 December 1916, Baker was engaged in battle with his unit near Gueudecourt.
During the action, he was posted as a telephonist with the forward observation team sent to record the fall of the artillery and secure the range for a bombardment.
Their position during this time was on a forward slope of the Australian frontline, which was subject to constant observation and the attention of German snipers.
Attempting to maintain communications, Baker ventured out on four occasions during the engagement, each time subject to the heavy artillery barrage from the German forces, and repaired the telephone line in thirty separate places.
As a consequence, the 16th Battery was able to align its artillery barrage and destroy the Germans' forward trench.
Commended for his "great gallantry" as well as "good service and ... great devotion to duty", Baker was awarded the Military Medal.
The announcement for the decoration was published in a supplement to the London Gazette on 19 February 1917.

In 1917, the 16th Battery was relocated to the Messines sector of the frontline in Belgium.
On 16 March, Baker was admitted to hospital suffering from an illness; he returned to his unit six days later.
During the afternoon of 21 June, the unit's position was subject to a severe bombardment of artillery shelling, resulting in an order for all men to evacuate the gun pits and seek cover.
As a consequence of the barrage, the camouflage covering one of the gun pits caught fire, endangering approximately 300 rounds of shrapnel and high explosives.
The battery sergeant major immediately called for men to assist him in quelling the blaze; Baker and three others volunteered.
Despite the continuous shellfire, the four men promptly set about retrieving water from a nearby well and shellholes with buckets "at great personal risk", dousing the fire.
The camouflage had been completely destroyed, several sand bags had caught fire, and a few rounds of ammunition were charred.
As a result of their actions, Baker's three companions were recommended for the Military Medal, and Baker for a bar to his.
The announcement of the decorations was promogulated in a supplement to the London Gazette on 21 August 1917.
On 29 July 1915, Baker enlisted in the Australian Imperial Force for service during the First World War. Allocated as a reinforcement to the 6th Field Artillery Brigade with the rank of gunner, he embarked from Melbourne aboard HMAT Persic on 22 November, bound for Egypt. On arrival, Baker was posted to the 16th Battery before moving to France for service on the Western Front. Disembarking in France on 1 July 1916, Baker took part in the Somme offensive.
On 11 December 1916, Baker was engaged in battle with his unit near Gueudecourt. During the action, he was posted as a telephonist with the forward observation team sent to record the fall of the artillery and secure the range for a bombardment. Their position during this time was on a forward slope of the Australian frontline, which was subject to constant observation and the attention of German snipers. Attempting to maintain communications, Baker ventured out on four occasions during the engagement, each time subject to the heavy artillery barrage from the German forces, and repaired the telephone line in thirty separate places. As a consequence, the 16th Battery was able to align its artillery barrage and destroy the Germans' forward trench. Commended for his "great gallantry" as well as "good service and ... great devotion to duty", Baker was awarded the Military Medal. The announcement for the decoration was published in a supplement to the London Gazette on 19 February 1917.
In 1917, the 16th Battery was relocated to the Messines sector of the frontline in Belgium. On 16 March, Baker was admitted to hospital suffering from an illness; he returned to his unit six days later. During the afternoon of 21 June, the unit's position was subject to a severe bombardment of artillery shelling, resulting in an order for all men to evacuate the gun pits and seek cover. As a consequence of the barrage, the camouflage covering one of the gun pits caught fire, endangering approximately 300 rounds of shrapnel and high explosives. The battery sergeant major immediately called for men to assist him in quelling the blaze; Baker and three others volunteered. Despite the continuous shellfire, the four men promptly set about retrieving water from a nearby well and shellholes with buckets "at great personal risk", dousing the fire. The camouflage had been completely destroyed, several sand bags had caught fire, and a few rounds of ammunition were charred. As a result of their actions, Baker's three companions were recommended for the Military Medal, and Baker for a bar to his. The announcement of the decorations was promogulated in a supplement to the London Gazette on 21 August 1917.
Thomas Baker (aviator)