An extraordinary man. Enlisted in the ranks as a private soldier and joined the 16th Lancers, November 1877. Became an NCO 1879. Offered a commission by his CO, he was gazetted Second Lieutenant in 3rd Dragoon Guards, June 1888. Spent next 4 years as a subaltern in India (during which time he became fluent in Hindi), before being attached to the Intelligence Branch at Simla, the HQ of the British Army in India. In 1894 as a Lieutenant he was appointed to the HQ staff of the Chitral Relief Force. January 1897 saw Robertson enter Staff College in England, being the first officer to have been promoted from the ranks to have done so. During this time he learned to speak French and extensively studied the areas of the Meuse Valley and the Belgian Ardennes (which of course became battlegrounds from the earliest days of the Great War). On passing Staff College he was appointed to the Intelligence Staff of the War Office, where he worked at times in the Russian and Colonial Sections. In 1895 he was promoted to Captain. He played an important part in the work of Intelligence during the early part of the South African War, and in December 1899 was ordered to join the Headquarters Staff of Lord Roberts, commanding the army in South Africa. By November 1900, he was promoted Brevet Lieutenant-Colonel for his work during the war, where he had also been Mentioned in Despatches. Robertson returned to England, where he resumed work in the Intelligence Staff of the War Office, where he was made Head of the Foreign Section. Promoted to full Colonel in 1904, by 1907 his appointment expired and he was placed on half pay. During this quiet period he undertook to translate German regulations regarding heavy artillery, before being appointed as Assistant Quartermaster-General at Aldershot (where at this time some 30,000 troops were based). In December 1907, he became Chief of the General Staff to Sir Horace Smith-Dorrien, who had succeeded Sir John French as Commander-in-Chief of the Aldershot garrison. Robertson recalled in his book 'From Private to Field-Marshal' how 'I would often, when passing in that direction as Chief of the General Staff, gaze at the old barrack-room where I first lived...and wonder how it had come about'.

In June 1910, Robertson was offered the post of Commandant of the Staff College, a most prestigious appointment that he held until October 1913. (He was promoted to Major-General too). He then rejoined the War Office as Director of Military Training, responsible for all training at home - including the establishments at Woolwich and Sandhurst - and for home defence. During war, the post automatically became Chief General Staff Officer for home defence.

When war broke out in August 1914, it appeared to Robertson that he had little chance of going to France due to his home defence role. At the last moment, however, Sir John Grierson - who had been nominated Chief of the General Staff of the BEF - was given command of II Corps, and Sir Archibald Murray was selected to succeed him. The appointment of Quartermaster-General of the BEF thus became available and it was offered to Robertson.

The Quartermaster-General's Staff dealt with supplies, transport and the issue of military stores. It clothes, feeds and houses the troops, and supplies them with all they need to fight - horses, motors, ammunition, guns, and other equipment. It also moves the troops, by rail and sea. He left London on the afternoon of 14th August, crossing from Dover on HM's Cruiser Sentinel, and landed at Boulogne. Robertson was responsible for supply through all of the early battles, from Mons to First Ypres. His idea to form dumps of supplies along the roadsides ensured that the BEF did not go short of basic supplies during the retreat.

In January 1915, the Commander-in-Chief asked Robertson to replace Murray as Chief of the General Staff, the responsible advisor on all matters affecting military operations. He remained in this position through the battles of 1915, as the BEF grew in size, experienced the trauma of shell and other shortages, and failed to succeed in several attempts to break through the German defences. On 23rd December of the same year he took up the new post of Chief of the Imperial General Staff, which was effectively the most senior role the army had to offer. This move came only after Robertson took care to define the role, which required a bargain with Kitchener. It took him back to London, into an unfamiliar role of dealing daily with the 'frocks' (politicians) who formed the War Cabinet and other bodies. He was promoted to Lieutenant-General in October 1915.

On 11th February 1918, he was told to leave the War Office. His position having been undermined by the Prime Minister, Robertson had made it clear that he was not prepared to continue. He was replaced by Sir Henry Wilson, a man who had played a not inconsiderable role in the political manoeuvring that created the so-called Supreme War Council to which Robertson objected. Robertson had been CIGs throughout the major battles of 1916 and 1917, arguably those that broke the German army.

On his return to England he was given the position of Commander-in-Chief of the Eastern Command, and after the war that of the British Army of the Rhine. On 29th March 1920 he was made Field-Marshal, the first man to have risen from the lowest to the most senior rank in the British Army.