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.