Macarthur-Onslow served as aide-de-camp to the Governor-General of Australia, the Earl of Hopetoun in 1902. He commanded the 2nd Light Horse Regiment from July 1903 until December 1907 when he was promoted to colonel on assuming command of the 1st Light Horse Brigade. He was on the unattached list in January 1910. Although too old for active service during the First World War, Macarthur-Onslow held the rank of colonel with the Sea Transport Service of the Australian Imperial Force, commanding troop ships travelling between Australia, the Middle East and Britain between 1915 and 1917. He was aide-de-camp to the Governor-General a second time from 1917 to 1920. He retired with the rank of major general in 1925.