A farmer and citizen soldier, during the late 1870s and early 1880s, Royston served in the Natal Mounted Rifles as an enlisted soldier and fought during the Zulu War. Later, during the Second Boer War, he was commissioned and fought at the Siege of Ladysmith, before later commanding a contingent of the Western Australian Mounted Infantry. Later he served during the Zulu Rebellion before organising the Natal Light Horse – made up primarily of Australians who had remained in Africa after the Boer War – upon the outbreak of the First World War.
After seeing action against the Germans in South-West Africa, Royston was transferred to Egypt and placed in command of the 12th Light Horse Regiment, commanding them through the Battle of Romani in 1916.
He was later promoted to command the 2nd Light Horse Brigade temporarily, before taking command of the 3rd Light Horse Brigade, and leading them in the Sinai and Palestine campaign against the Ottoman Empire until October 1917 when he returned to South Africa having been relieved of his command due to medical reasons.