Hamilton attended the Royal Military College, Sandhurst in 1869, the first year that entrance to the British Army as an officer was regulated by academic examination rather than by the monetary purchasing of a commission.

In 1871 he received a commission as an infantry officer with the Suffolk Regiment, but shortly afterwards transferred to the 2nd Battalion of The Gordon Highlanders, who at that time were on Imperial garrison service in India.

He took part in the Chitral Expedition as military secretary to Sir George Stuart White, commander in chief of forces in India.

Hamilton was given command of the southern sector of the town's defences and successfully fought off the only major assault on the garrison at the Battle of Wagon Hill in January.

In May 1901 Hamilton was appointed Military Secretary at the War Office,[5] but the following November he was again asked to return to South Africa as Chief of Staff to the Commander-in-Chief, Lord Kitchener.

[8] Following the end of hostilities in June 1902, he returned to the UK together with Lord Kitchener on board the SS Orotava,[9] which arrived in Southampton on 12 July.

I am much indebted to him for his able and constant support to me as Chief of Staff, also for the marked skill and self-reliance he showed later, when directing operations in the Western Transvaal.

[12] He returned to his post as Military Secretary at the War Office in September 1902,[13] and the same month accompanied Lord Roberts, Commander-in-Chief of the Forces, and St John Brodrick, Secretary of State for War, on a visit to Germany to attend the German army maneuvers as guest of the Emperor Wilhelm.

Conversely, the successful Japanese infantry assaults convinced him that superior morale would allow an attacker to overcome prepared defensive positions.

In March 1915, Lord Kitchener appointed Hamilton, aged 62, to the command of the Allied Mediterranean Expeditionary Force, with orders to gain control of the Dardanelles straits from the Ottoman Empire and to capture Constantinople.

While a senior and respected officer, perhaps more experienced in different campaigns than most, Hamilton was considered too unconventional, too intellectual, and too friendly with politicians to be given a command on the Western Front.

While the high command of the Greek Army, which possessed far more detailed knowledge of the Ottoman Empire's military capacity, warned Kitchener that a British Expeditionary Force entering the Eastern Mediterranean theatre would require 150,000 troops to capture Gallipoli, Kitchener concluded that a force of 70,000 men would be adequate to overpower any defensive garrison there.

In November of that year, ships from the Royal Navy had shelled its outer forts, causing the magazine at Seddülbahir castle to explode.

Winston Churchill, as First Lord of the Admiralty, had initially suggested in September 1914 that the operation would need the support of only 50,000 men, a strength of just over two British Army divisions.

Starting on 19 February 1915, British and French warships attempted to take the strait using naval power alone but failed after an abortive attack foundered upon sea mines.

[26] With the Gallipoli Campaign stalled, Hamilton was recalled to London on 16 October 1915, effectively ending his military career.

In retirement, Hamilton was a leading figure in the ex-servicemen organization, the British Legion, holding the position of Scottish President.

Hamilton traveled personally to the German capital and received the Gordon drums in a ceremony in which he spoke of his wish that there will be "no more war" between the "old allies of Waterloo.

Examples of his written works include: The Fighting of the Future, Icarus, A Jaunt on a Junk, A Ballad of Hadji, and A Staff Officer's Scrapbook.

[36] British Prime Minister H. H. Asquith remarked that he thought that Hamilton had "too much feather in his brain",[37] whereas Charles Bean, a war correspondent who had reported from the scene of the Gallipoli campaign in 1915 and who went on to write Australia's Official History of the 1914–1918 War, considered that Hamilton possessed "a breadth of mind which the army in general does not possess".