Educated at Eton College, Methuen served two years as a cornet in the Royal Wiltshire Yeomanry and then joined the Scots Fusilier Guards as an ensign in the regiment and lieutenant in the army on 22 November 1864. He was promoted to lieutenant in the regiment and captain in the army on 25 December 1867, and became adjutant of the 1st battalion in 1868. He became brigade major, Home District in 1871 and saw active duty on the staff of Sir Garnet Wolseley at Amoaful in 1873 during the Third Anglo-Ashanti War. Promoted to captain in the regiment and lieutenant colonel in the army on 15 July 1876, he became assistant military secretary in Ireland in 1877, military attaché in Berlin in 1878 and quartermaster-general at the Home District in April 1881, before being promoted to colonel on 1 July. He was the commandant of headquarters in Egypt for three months in 1882, being present at the Battle of Tel el-Kebir. On return to the UK he became assistant-adjutant and quartermaster-general for the Home District again. He was promoted to major in the regiment on 25 October 1882.
Methuen served in the expedition of Sir Charles Warren to Bechuanaland in 1884 to 1885, where he commanded Methuen's Horse, a corps of mounted rifles. He became deputy adjutant-general, in South Africa in 1888, and having been promoted to major general on 21 May 1890, he succeeded his father as 3rd baron in 1891. He became Major-General commanding the Brigade of Guards and General Officer Commanding the Home District in April 1892 and then served as press censor at headquarters on the Tirah expedition in 1897. Promoted to lieutenant general on 1 April 1898, he was given the command of the 1st Division on the outbreak of the Second Boer War.
Methuen reached South Africa in November 1899 with orders to relieve Kimberley but initially just expelled the Boers from Belmont and Graspan. He was slightly wounded at the Battle of Modder River. He suffered both defeats and successes during the war. His greatest defeat was at the Battle of Magersfontein, during which he failed to carry out adequate reconnaissance and accordingly his artillery bombarded the wrong place leading to the Highland Brigade taking heavy casualties. The battle was regarded as one of the three British disasters in "Black Week" that led to the despatch of Lord Roberts to South Africa.