 Cheyne served during the Boer War as a consulting surgeon for the British military in South Africa from 1900 to 1901.In a despatch dated 31 March 1900, the Commander-in-Chief in South Africa, Lord Roberts, described how Cheyne had "rendered invaluable service by … advice and assistance to the Medical Officers" and "been unwearying in … work among the wounded and sick".In 1910 he was made Honorary Surgeon-in-Ordinary to King George V. With the outbreak of World War I, he became a consulting surgeon to the Royal Navy in 1914, and in 1915 was for a short time temporary Surgeon General, RN.