Leachman was commissioned a second lieutenant in the Royal Sussex Regiment on 20 January 1900, and the following month left with his battalion for service in South Africa during the Second Boer War. He served there until the end of the war, in June 1902, and left Cape Town in the SS Bavarian in August, returning to Southampton the following month. He later served in India, but spent most of his career as a political officer in Mesopotamia, where he was instrumental in pacifying warring tribes to bring stability to the new country. Leachman also made various expeditions further south into Arabia, where he contacted Ibn Sa'ud on behalf of the British government. He travelled as a naturalist of the Royal Geographical Society, but was in fact a British agent.