1866, and captain on 16 Feb. 1878. He served with the second battalion of his regiment in the operations against Sandile in Kaifraria in 1878, and in the Zulu war of 1879, receiving the medal with clasp. Owing to the destruction of the first battalion at Isandhlwana, he obtained his majority on 1 July 1881. He went to India with his battalion in 1880, and on 30 Sept. 1882 was appointed assistant adjutant-general for musketry in Madras. He served on the staff in the expedition to Burma in 1885, and afterwards organised a force of mounted infantry which won special praise from Lord Roberts (Forty-one Years in India, p. 518). In 1889 he commanded the Burma column in the Chin-Lushai expedition, and received the thanks of the Indian government. He was repeatedly mentioned in despatches (London Gazette, 22 June 1886, 2 Sept. 1887, 15 Nov. 1889, 12 Sept. 1890), and was given the brevet rank of lieutenant-colonel (17 May 1886) and of colonel (1 July 1887), the C.B. (14 Nov. 1890), and the Indian medal of 1894 with two clasps.
On 31 Jan. 1891 he was promoted regimental lieutenant-colonel, and commanded the second battalion of the South Wales borderers (late 24th) till 8 April 1893, when he became, by Lord Roberts s selection, assistant adjutant-general for musketry in Bengal. An excellent shot and a skilful swordsman himself, he did his best to raise the standard of shooting in the army. On 25 March 1895 he was appointed to command a second-class district in the Punjab as brigadier-general. He commanded a brigade in the Waziristan expedition of 1894-6 (ib. 2 July 1895), and received the clasp. In 1898 he commanded a brigade in the Tochi field force, and afterwards the first division in the Tirah expedition (ib. 11 Feb. and 5 April 1898). He was made K.C.B. on 20 May 1898, and received the Indian medal of 1895 with two clasps.
On 15 May 1899 he was appointed to the command of the troops in Natal, then numbering about five thousand men. War with the Transvaal Republic was already in prospect, and in July Symons informed the governor that an increase of sixteen hundred men was required to defend the colony against raids, and of 5,600 men to defend it against an invasion. In the autumn reinforcements larger than he had asked for came from India and the Mediterranean, and on 20 Sept. Symons was given the temporary rank of major-general. To meet the wish of the civil government of Natal, he divided his troops between Ladysmith and Dundee. On 3 Oct. Sir George White arrived and assumed the chief command in Natal. War was declared by the Transvaal and the Orange Free State Republics on 10 Oct. The troops were organised as the fourth division of the South Africa field force, under Symons who was made temporary lieutenant-general on 9 Oct. He was sent to Dundee, where four battalions, three batteries, and one cavalry regiment were encamped. There he was attacked on 20 Oct. by about four thousand Boers with six guns under Lucas Meyer. These had come from the east, while two other bodies were approaching, from the north and west, blocking the railway from Ladysmith. The guns of Meyer's force opened fire on the camp at daybreak from Talana hill, three miles to the east of it. Symons led out his troops and assailed this hill with three battalions. By 1.30 P.M. it was most gallantly stormed, but Symons was mortally wounded by a bullet in the stomach in the course of the advance. Two days afterwards the British force retired on Ladysmith, but Symons, with other wounded men, had to be left at Dundee, and he died there on the 23rd. He was buried on the 24th in the church of England burial-ground, with marks of respect from the Boers. The 'London Gazette' of that day notified his promotion to major-general for distinguished service in the field. Sir George White described him as 'an officer of high ability and a leader of exceptional valour.' A memorial window in Botusfleming Church, near Saltash, Cornwall, was unveiled in October 1900.
On 13 Feb. 1877 he married Caroline, only daughter of Thomas Pinfold Hawkins of Edgbaston ; she survived him.
[Burke's Landed Gentry; Historical Records of the 24th Eegiment (of which Symons was one of the editors); Hutchinson's Campaign in Tirah; Parliamentary Papers, Cd. 44, correspondence relative to the defence of Natal; Standard, 27 Oct. 1899.]