1788, ad fin.], by his wife Anna Maria Le Grand, was born at his father's residence, Roath Court, near Cardiff, 24 April 1805. His family in 1815 removed to Guernsey. Jacob was educated at Elizabeth College, Guernsey, and under private tutors in France and England, and when about fifteen was sent to London to learn oriental languages under Dr. John Borthwick Gilchrist [q. v.] He obtained an Indian infantry cadetship in 1820, and on the voyage out to Bombay contracted a close friendship with Alexander Burnes [q. v.] He was posted to the 2nd or grenadier regiment Bombay native infantry (now Prince of Wales's own) as ensign 9 June 1821, in which corps he obtained all his regimental steps except the last. His subsequent commissions were: lieutenant 10 Dec. 1823, captain 6 June 1836, major 1 May 1848, lieutenant-colonel in the (late) 31st Bombay native infantry 15 Nov. 1853, brevet-colonel 6 Dec. 1856, brigadier-general 21 July 1858, major-general on retirement 31 Dec. 1861.
Jacob passed for interpreter in Hindustâni so speedily after arrival in India, that he was complimented in presidency general orders. He afterwards passed in Persian and Marâthi. He saw some harassing service with his regiment against the Bheels in the pestiferous Nerbudda jungles, and was subsequently with it in Cutch and at Ukulkote. He took his furlough home in 1831, and in January 1833 was appointed orderly officer in the East India Military Seminary, Addiscombe. While there, at the request of the Oriental Translation Fund, he undertook the translation of the ‘Ajaib-al-Tabakat’ (Wonder of the Universe), a manuscript purchased by Alexander Burnes in the bazaar at Bokhara. Jacob considered the work not worth printing, and his manuscript translation is now in the library of the Asiatic Society, London. On 18 June 1835 he married Emily, daughter of Colonel Utterton of Heath Lodge, Croydon, and soon afterwards sailed for India. His wife died at sea, and Jacob landed at Bombay in very broken health. He recovered under the care of a brother, William Jacob, then an officer in the Bombay artillery, and in 1836 was appointed second political assistant in Kattywar, where he was in political charge in 1839–43. His ability in dealing with the disputed Limree succession was noticed by the government; the curious details are given in his book (Le Grand Jacob, Western India, pp. 22–55). He was also thanked for his report on the Babriawar tribes (1843) and other reports on Kattywar. Early in 1845 he served as extra aide-de-camp to Major-general Delamotte during the disturbances in the South Mahratta country, and was wounded in the head and arm by a falling rock when in command of the storming party in the assault on the hill-fort of Munsuntosh. In April 1845 Jacob was appointed political agent in Sawunt Warree. The little state was bankrupt, with its gaols overflowing; but Jacob's judicious measures during a period of six years restored order, retrieved the finances, and reformed abuses. On 8 Jan. 1851 Jacob was made political agent in Cutch, and was sent into Sind as a special commissioner to inquire into the case of the unfortunate Mir Ali Morad, khan of Khypore, the papers relating to which were printed among ‘Sessional Papers’ of 1858 and the following years. He also sat on an inquiry into departmental abuses at Bombay. An account of his travels in Cutch appeared in the ‘Proceedings’ for 1862 of the Bombay Geographical Society, since merged in the Asiatic Society of Bombay. His health needing change, he obtained leave, and visited China, Java, Sarawak, and Australia, ‘keeping his eyes and ears ever on the alert, always reading, writing, or inquiring—mostly smoking—winning men by his geniality and women by his courteous bearing’ (Overland Mail, 6 May 1881). On his return he was shipwrecked on a coral reef in Torres Straits, and saved from cannibal natives by a Dutch vessel. He quitted Cutch for Bombay in December 1856, at first purposing to retire; but he served under Outram in the Persian expedition. In Persia he was in command of the native light battalion in the division under Henry Havelock, whom Jacob appears to have regarded as too much of a martinet. He returned with the expeditionary force to Bombay in May 1857.
Acting under the orders of Lord Elphinstone, the governor of Bombay, Jacob arrived at Kolapore on 14 Aug., a fortnight after the 27th Bombay native infantry had broken into mutiny there. Four days later he, with a mere handful of troops, quietly disarmed the regiment, and brought the ringleaders of the outbreak to justice ({{sc|Jacob,, Western India, pp. 144–77). On 4 Dec. following, when the city closed its gates against Jacob's small force which was encamped in their lines outside, Jacob promptly blew open one of the gates, put the rebels to flight, tried by drumhead court-martial and executed on the spot thirty-six who were caught red-handed, and held the city until the mischief was past (ib. pp. 182–208). His vigour, no doubt, prevented the wave of rebellion from sweeping over the whole southern Mahratta country and overflowing into the nizam's dominions (Holmes, Indian Mutiny, p. 455; Report on Administration of Public Affairs in Bombay,
