He served in Egypt and Sudan for several years during the Mahdist War; he was Aide-de-camp (ADC) to General Sir Redvers Buller during the Suakin Expedition in 1884, where he took part in the battles of El Teb (February 1884) and Tamai (March 1884), and for which he was mentioned in despatches, received the medal with clasp, and the bronze star.
Taking part in the Nile Expedition 1884–1886, he was ADC to Major-General Earle in 1883–1885 and after his death to General Henry Brackenbury in 1885, and was present at the Battle of Kirbekan (February 1885), for which he was again mentioned in despatches and received claps to the medal.
[4] Transferring from Africa to Asia, he was ADC to Sir William Des Vœux, Governor of Hong Kong, from 1889 to 1890, and was promoted captain on 1 July 1890 and brevet major the following day.
He was in charge of more than 1100 officers and men of the guards regiments which returned to the United Kingdom on board the SS Lake Michigan in October 1902, following the end of the war earlier that year.
[6][7] St Aubyn commanded the Grenadier Guards from 1904 to 1908, during which he was promoted to colonel in 1905, and retired from the regular army in 1908 after he inherited the barony from his father.