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 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.
Leachman's first major expedition south into the Arabian Peninsula was in 1909, during which he was involved in a ferocious battle between the Anaiza and Shammar tribes near Ha'il. He was awarded Macgregor Memorial medal for reconnaissance in 1910. In 1912 Leachman made a second expedition with the intention of crossing the Rub Al Khali, but was refused permission by Ibn Sa'ud when he reached Riyadh and instead went to Hasa. He was the first Briton to be received by Ibn Sa'ud in his home city.
In December, 1915, during the Siege of Kut, the British commanding officer, Major General Charles Townshend, ordered Leachman to save the British cavalry by breaking out and riding south. This he did and the cavalry were the only British unit to escape before the fall of the city to the Ottomans.
Prior to the conclusion of the war, Leachman was assigned to the 17th Division, which was assigned the task of operating on both the left and right banks of the Tigris in an effort to advance north in order to secure as much territory from the Ottomans prior to the now inevitable surrender of the Ottoman Empire. Leachman was specifically assigned to Light Armoured Motor Brigade on the right bank of the Tigris, ostensibly with a special task to work with local tribes.
After the war, he was assigned as Political Officer for the Mosul Division within Mesopotamia, up until October, 1919. He was murdered during the 1920 insurrection by a son of Sheikh Dhari, from the Zoba tribe, near Fallujah on 12 August 1920. Leachman had visited Dhari in an effort to renegotiate repayment of advances made to him by the government and to persuade him to remain loyal to the current administration, but was shot in the back by Dhari's son after a verbal disagreement over a local robbery. Leachman's death sparked an immediate outbreak of tribal uprisings on the Euphrates between Falluja and Hit, and was responsible for General Haldane's advance on the same area in September, 1920. He was buried in the British Military Cemetery in Baghdad.
He was played by Oliver Reed in Al-Mas' Ala Al-Kubra (aka Clash of Loyalties), a 1983 film financed by Saddam Hussein, which was nominated for the Golden Prize at the 1983 Moscow International Film Festival.
- "The War - Embarcation of Troops". The Times (36063). London. 12 February 1900. p. 10.
- Omer Tarin and SD Najumddin, 'G.E Leachman and the MacGregor Memorial Medal:Revaluation of the Life and Work of an Arabian Paladin', in Durbar: Journal of the Indian Military Historical Society, UK, 2 parts: Vol 25 No 3, Autumn 2008, pp. 116-125; and Vol 25, No 4, pp. 174-184
- Tarin and Najumddin, Vol 25, No 4, pp. 176-179
- Wilson, Sir Arnold. Mesopotamia 1917-1920: A Clash of Loyalties. London: Oxford University Press, 1931. p. 11
- Hay, W. R., Two Years in Kurdistan: Experiences of a Political Officer 1918-1920. London: Sidgwick & Jackson, LTD, 1921. P 179.
- Hay, p. 367.
- Wilson, p.292
- Wilson, p. 293
- "IMDB - awards". Retrieved 14 February 2009.
- A Paladin of Arabia. The Biography of Brevet Lieut.-Colonel G. E. Leachman, N.N.E.Bray, Unicorn Press (1936). ISBN 0-7103-0976-7
- Travellers in Arabia, Eid Al Yahya, Stacey International (2006). ISBN 0-9552193-1-0 (9780955219313)
- OC Desert; The Life of Lieutenant-Colonel Gerard Leachman, H.V.F. Winstone, Quartet (1982). ISBN 0-7043-2330-3
- Hay, W.R. Two Years in Kurdistan: Experiences of a Political Officer 1918-1920. London: Sidgwick & Jackson, Ltd., 1921.
- Wilson, Sir Arnold. Mesopotamia 1917-1920: A Clash of Loyalties. London: Oxford University Press, 1931.