Edinburgh University, where he won the mathematical medal for his year, he entered the East India Company's military college at Addiscombe in February 1847. He passed out at the head of his term, was awarded the Pollock medal (presented to the most distinguished cadet of the outgoing term), and was commissioned as 2nd lieutenant in the Bengal engineers on 8 Dec. 1848.
After passing through the usual course at Chatham, Innes arrived in India in November 1850. He was at first employed in the Public Works Department on the construction of the Bari Doab canal in the Punjab. On 1 Aug. 1854 he was promoted lieutenant, and in 1857, shortly after the annexation of Oude, he was transferred to that province as assistant to the chief engineer.
When the Mutiny began in May 1857 Innes was at Lucknow. He was given charge of the old fort the Machi Bhowan, with orders to strengthen it, so that it would both overawe the city and serve as a place of refuge. After the siege began in June the disastrous action of Chinhut made it necessary to concentrate the whole of the garrison at the Residency. Orders were given for the evacuation of the Machi Bhowan and Innes, one of the most fearless and energetic of the subalterns, assisted to blow it up. On the morning of 20 July the rebels assembled in large masses and exploded a mine in the direction of the Redan battery, leaving an enormous crater. They advanced boldly to the assault, but Lieutenant Loughman in command, with Innes and others, drove them back after four hours' fighting.
Innes was especially employed in mining. On 21 Aug. after sixty-four hours' hard work and no sleep he blew up Johannes's house, from which the rebel sharpshooters had fired with deadly effect and had practically silenced a British battery. During the relief by General Havelock Innes took part in all the sorties, and after the general had entered the city on 25 Sept. 1857, he was placed in charge of the mining operations in the new position occupied by Havelock's force in the palaces on the bank of the river. The defence was then chiefly confined to mining and counter-mining until the final relief by Sir Colin Campbell on 22 Nov. Innes's book, entitled 'Lucknow and Oude in the Mutiny' (1895), stands almost alone for sobriety and balance among accounts of the defence of Lucknow and the operations in Oude.
After the evacuation of Lucknow, Innes was posted to Brigadier-general Franks's division, and during its march through Oude he was present at the affairs of Miratpur, Chandi and Amirpur. He greatly distinguished himself at the battle and capture of Sultanpur on 23 Feb. 1858. For a splendid act of gallantry during the advance in putting out of action by his single-handed boldness a dangerous gun of the enemy General Franks recommended him for the Victoria Cross, observing that his courage was 'surpassed by none within his experience.' Subsequently on 4 March, the day on which Franks effected his junction with Sir Colin Campbell to besiege Lucknow, Innes was severely wounded at the attack on the fort at Dhowrara, eight miles from Lucknow. He was promoted 2nd captain on 27 Aug. 1858.
For his services in the Lidian Mutiny Captain Innes was three times mentioned in despatches; he received the brevet rank of major on 28 Aug. 1858, the Victoria Cross, the medal with two clasps, and a year's service for the defence of Lucknow. When the military college at Addiscombe was closed in Juno 1861, the secretary of state for India, in addressing the last batch of cadets, read out Lord Canning's speech on presenting Innes with the Victoria Cross. After the Mutiny campaign Innes was appointed garrison engineer at Fort William, Calcutta; he then served in various grades of the public works department in the central provinces and in the Punjab until 1867. In the following year he was appointed a member of the commission to investigate the failure of the bank of Bombay. In 1869 he started the upper section of the Indus valley railway, and in the following year he was appointed accountant-general of the public works department, and held that important post for seven years. In the meantime his military promotion had run on. He was promoted 1st captain in his corps on 29 Feb. 1864; brevet lieut.-colonel on 14 June 1869; regimental major on 5 July 1872; regimental lieut.-colonel on 1 April 1874; and brevet colonel on 1 Oct. 1877.
In 1882 Innes was appointed inspector-general of military works. He was a member of the Indian defence committee, and many new defences were carried out under his orders. He was promoted major-general on 28 Nov. 1885, and retired from the service with the honorary rank of lieut.-general on 16 March 1886. On the jubilee celebration of the defence of the Residency at Lucknow in June
