Born in Brackenhurst, he was educated at Haileybury College and the Royal Military College, Sandhurst. In 1882 he was commissioned into the 6th Inniskilling Dragoons. He entered active service in 1884 on the Bechuanaland Expedition, and later in Zululand (1888). He returned to Britain with his unit in 1890 and in 1896-97 completed the course at the Staff College, Camberley. He was promoted to Major in 1897 and in 1898 joined the 3rd Cavalry Brigade, then serving in Ireland. He returned to southern Africa to fight in the Second Boer War from 1899. He finished the war as a Colonel and returned to Britain in 1902 to command the 5th Royal Irish Lancers until 1905 and then the 4th Cavalry Brigade until 1910. His extensive cavalry experience led to him being made Inspector of Cavalry.
During World War I he initially served on Western Front. At the outbreak of war he was made commander of a cavalry division and distinguished himself when his unit covered the retreat after the Battle of Mons. He was rewarded by being made commander of the BEF Cavalry Corps. In 1915 he commanded V Corps during the Second Battle of Ypres and in October he took charge of the Third Army. However at the Battle of Arras, his forces failed to expolit a breakthrough and he was replaced by Julian Byng on June 9.
Allenby was sent to Egypt to be made commander-in-chief of the Egyptian Expeditionary Force on June 27, 1917, replacing Sir Archibald Murray. One of Allenby's first moves was to support the efforts of T. E. Lawrence amongst the Arabs with £200,000 a month. Having reorganised his regular forces Allenby won the Third Battle of Gaza (October 31-November 7, 1917) by surprising the defenders with an attack at Beersheba. His force pushed on towards Jerusalem, the Ottomans were beaten at Junction Station (13-15 November) and Jerusalem was captured on December 11, 1917. Allenby entered the city on foot through the Jaffa Gate.
The German offensive on the Western Front meant that Allenby was without reinforcements and after his forces failed to capture Amman in March and April 1918 he halted the offensive. New troops from the Empire (specifically Australia, New Zealand, India and South Africa) led to the resumption of operations in August 1918. Following an extended series of deceptive moves the Ottoman line was broken at the Battle of Megiddo (September 19-21, 1918) and the Allied cavalry passed through and blocked the Turkish retreat. The EEF then advanced at an enormous rate, encountering minimal resistance, Damascus fell on October 1, Homs on October 16, and Aleppo on October 25. Turkey capitulated on October 30, 1918
Allenby was made a Field Marshal in 1919 and on August 6 was created Viscount of Megiddo. He remained in the Middle East as High Commissioner for Egypt and the Sudan until 1925 and he was instrumental in the creation of sovereign Egypt. He retired in 1925 and died in London.