Thorneycroft was commissioned a
second lieutenant in the
Royal Scots Fusiliers on 22 February 1879, and served in the
Anglo-Zulu War later the same year, followed by the operations against
Sekukuni. Two years later, he served in the
First Boer War of 1881, and took part in the defence of
Pretoria, following which he was promoted to
lieutenant on 1 July 1881. He was promoted to
captain on 23 January 1887, to
major on 1 July 1899, and received the
brevet rank of
lieutenant colonel on the same day. Expecting military hostilities, Thorneycroft was one of several officers sent to South Africa to raise volunteer units shortly before the
Second Boer War broke out in October 1899. He raised a corps of troopers, later known as the Thorneycroft's Mounted Infantry, which served in the early stages of the war, including in the campaign to
relieve Ladysmith, which was under siege by troops from the Boer republics. In the
Battle of Spion Kop in late January 1900, he was selected to lead the initial assault. Before dawn, the British forces had captured what they thought was the summit of the kop, or hill. The Boers, who actually held the higher ground, soon counter-attacked, swarming the British position. The higher ranking British officers were killed or mortally wounded, leaving Thorneycroft the most senior officer present. A British counterattack failed in the face of withering fire from the Boers, but Thorneycroft refused to allow any under his command to surrender. Not knowing that they actually had gained the upper hand over the course of the day, and running short on both water and ammunition, the British troops retreated under cover of dark. The battle at Spion Kop was considered a resounding defeat, but the British regrouped and were able to relieve Ladysmith four weeks later. Thorneycroft received a
brevet promotion to lieutenant colonel and was appointed a
Companion of the Order of the Bath in the April 1901 South Africa Honours list (the award was dated 29 November 1900). He stayed in South Africa throughout the war, which entered a phase of guerrilla warfare from late 1900. In his final
despatch from South Africa in June 1902,
Lord Kitchener, Commander-in-Chief of the forces during the latter part of the war, described Thorneycroft as "an absolutely reliable officer of great experience, common-sense and force of character." For his service in the later part of the war, Thorneycroft received a brevet promotion to
colonel on 22 August 1902. After the end of the war in June 1902, Thorneycroft was among a number of officers who left
Cape Town on the in late July, arriving in
Southampton the following month. Following his return, he was in November 1902 appointed
assistant adjutant-general of the
7th Infantry Division, serving in
Dublin, and received the substantive rank of colonel on 12 November 1902. He was promoted to temporary brigadier general and became commander of the
14th Infantry Brigade in November 1905 and, promoted in September 1909 to major general, and became
general officer commanding (GOC)
South Midland Division in September 1911 before retiring from the army in July 1912. ==Later life==