Robert Burnside Carter left Sydney on 22 March 1915 to join the Royal Army Medical Corps in England with several other young Australian doctors. His brother, Lieutenant Colonel Herbert Gordon Carter also served in the First World War with the Australian Imperial Force. A doctor by profession, Carter was made a temporary lieutenant on 23 March 1915.
Sometime after its formation in September 1915, he was transferred to the Royal Field Artillery of the 123rd Brigade of the British Army as its regimental medical officer. Along with the 122nd and 124th Brigades, the 123rd Brigade was part of the 41st Division, a new division formed at the beginning of the war that was deployed to France in May 1916. The division participated in the Battles of the Somme (1916), the Battle of Messines (1917), the Battles of Ypres (1917), the First Battles of the Somme (1918), the Battles of the Lys (1918), and the final advance in Flanders (1918). Robert Carter's award of the Military Cross was published in the London Gazette on 3 June 1919.