After home service in Britain and Ireland his regiment, 9th Queen's Royal Lancers, was posted to South Africa in 1896, and on to India in 1898, though Campbell seems to have spent some of this period in the United Kingdom. The regiment returned to South Africa following the outbreak of the Second Boer War, during which he saw his first action. By the outbreak of the First World War he was in command of the regiment. He led them in two cavalry charges in the first months of the war; the second of these saw him receive multiple wounds, one of them from a lance, making him one of the last British casualties of that weapon. In November 1914 he was promoted to command 6th Cavalry Brigade, then in May 1916 came further promotion to command 21st Division. He retained that command for the rest of the war.
Post-war, he was knighted in 1919, and went on to hold further command and administrative positions in the army. He was General Officer Commanding (GOC) Baluchistan, 1920–24; Military Secretary, 1926–27; and GOC Aldershot Command 1927–31, when he argued for the increased mechanisation of the army. He was then appointed Governor of Malta, and died shortly after resigning that office in 1936.
During the early years of his military career Campbell was also a successful amateur National Hunt jockey, winning a number of major races; foremost amongst these was the Grand National of 1896, on The Soarer, which gave him his nickname in subsequent years (though his divisional staff referred to him as Barbara, for reasons now lost). He continued riding in military meets until the end of his army service. He was also a keen cricketer and polo player.
Campbell was born on 28 January 1869, the son of Major H. Wooton Campbell, Cameron Highlanders. He was educated at Clifton College where he was a noted sportsman, in particular playing cricket for the First XI. He then proceeded to Royal Military College, Sandhurst, also representing them at cricket, before he was gazetted a second lieutenant in the 9th Lancers on 15 March 1889. Joining the cavalry was an expensive business. A junior officer's pay was just £95 a year, but the expenses of regimental living—such as mess bills—meant that a private income of at least £500 a year was required, in addition to a similar amount when first commissioned to buy horses, uniform and other equipment. At the time the regiment was based in Manchester, with a detachment at Seaforth Barracks, Liverpool. In April 1890 there was a major reorganisation of British Army cavalry regiments, with the establishment being increased from 488 Other Ranks and 300 horses to 628 OR and 350 horses, followed in August by a further increase to 706 OR and 424 horses. August also saw the regiment move to Ireland, initially to the Curragh, and then Dundalk; due to the reorganisation, the regiment was now seriously understrength, so a recruitment campaign was started. Campbell was promoted lieutenant on 9 December 1890. Cavalry regiments, in addition to their increase in size, also had their structure reorganised, with the main sub-unit becoming the squadron rather than the troop, each squadron being formed by the merger of two troops. The remainder of their time in Ireland was split between the Curragh and Newbridge. In addition to the restructuring, the regiment was issued with new-style equipment in August 1893.
The regiment's tour in Ireland lasted until August 1894 when it was posted to Aldershot; the regimental history describes the period as "one of the most peaceful and undisturbed in that country before or since". Over this time, Campbell had begun to make a name for himself as an amateur jockey, and on 9 March 1894 at the Grand Military Meeting at Sandown Park he won the Maiden Steeplechase on The Soarer, a horse he had acquired on the toss of a coin. Later in the year, on 27 November he won the Middlesex Steeplechase and 28 November he won the Uxbridge Handicap Steeplechase Plate, both at Kempton Park, followed by third in the Handicap Steeplechase back at Sandown Park on 7 December 1894, and back at Kempton Park, another win in the Hampton Steeplechase Handicap on Boxing Day; all these races were also on Soarer. 1895 began rather less successfully, with a fourth on Soarer in the Grand Military Gold Cup at Sandown Park on 8 March, and the next day he failed to finish in the Grand Military Handicap Steeplechase, riding Seaside, which belonged to a fellow 9th Lancers officer. Then on 30 March, at the Liverpool Spring Meeting at Aintree, he won the Champion Steeplechase, again on Soarer. A single day in 1895 also saw him win both the Irish National Hunt Cup (on Dakota) and the Irish Grand Military (on Balbrigan).