In 1900 Macarthur-Onslow made his own way to South Africa in 1900 to participate in the Second Boer War, where he served as aide-de-camp to Hutton, who was now a major general. He participated in operations in the Orange Free State from February to May 1900, including the actions at Vet River and Zand River, and operations in the Transvaal from May to November 1900. For this service, he was mentioned in despatches by Field Marshal Lord Roberts. He returned to Australia in March 1901, but was later appointed to command the 5th Battalion, Australian Commonwealth Horse.