After being educated at Harrow School, Montagu was gazetted ensign in the 35th foot (27 October 1787) and then lieutenant in the 76th foot (25 December 1787). He also held a commission in the 50th foot from January 1788 to May 1790, and exchanged into the 73rd regiment on 29 February 1792. He attained the rank of colonel in the army on 14 March 1794, having been gazetted colonel of the Huntingdonshire militia on 8 March of the preceding year. His youth and early manhood seem to have been passed in travel and field sports. He specially excelled as a rower, and is said to have pulled a wherry from London to Gravesend without a rest.
On 7 October 1793 Manchester married Lady Susan Gordon (1774–1828), the third daughter ofAlexander Gordon, fourth duke of Gordon, and his wife, Jane Maxwell. They had two sons and six daughters before the marriage broke down; the duchess of Manchester was reported to have run off with one of her footmen some years before their final separation in 1813. In 1808 Manchester was made governor of Jamaica, where he arrived, without his wife, in March. The nineteen years of his government of the colony were times of great distress and anxiety. Two months after his arrival, on 30 May 1808, a mutiny of the 2nd West India regiment, a black corps, led to a quarrel between Carmichael, the commander-in-chief, and the colonial assembly. Manchester applied to the home authorities, and prorogued the assembly when it ordered Carmichael into custody. Five months later the general, under orders from the crown, apologized to the assembly, and Manchester's discretion was generally commended.
In 1811 Manchester paid a visit to England, but returned to Jamaica in 1813. During the following year attempts were made to reform the law courts and Post Office by fixing the amount of all fees, and a law was passed allowing ‘free people of colour’ to give evidence, but precluding them from holding offices. In 1815 Manchester sought to alleviate the distress caused by the destruction of Port Royal by fire on 13 July, and by the hurricanes and floods which destroyed the sugar and coffee plantations of the island on 18 and 19 October. He showed great administrative ability during the panic which prevailed in the colony following an insurrection of slaves in Barbados, and by his personal influence pacified the Jamaica slaves. The colony gratefully voted him an addition to his personal establishment. In 1816 he risked his popularity with the planters by vigorously supporting a bill for the registry of slaves, in accordance with the recommendation of the imperial government.
In 1820 Manchester was thrown from his carriage and fractured his skull. The assembly voted 500 guineas to the surgeons who attended him. After recuperating in Europe, Manchester returned in 1822, and the last years of his administration were marked by the introduction of measures preparatory to the emancipation of the slaves, which the planters solidly resisted. The Jamaica government was called upon by the Colonial Office to abolish Sunday markets, to forbid the carrying of whips, and to exempt women from flogging. All these reforms were carried out with great difficulty. In 1824 there was a slave insurrection in the west of the island, and a plot was apparently discovered for the massacre of the white inhabitants in the north and east. In 1825 the assembly rejected a bill allowing slaves to give evidence, but in the following year Manchester succeeded in securing a temporary measure to be in operation for five years. In this form, however, the law was vetoed by the home government, but before the imperial decision was known a conviction for murder was obtained by the evidence of slaves given under the temporary law. In the midst of the consequent confusion Manchester finally left Port Royal on 2 July 1827.
Soon after his return to England, on 27 September 1827, Manchester was appointed postmaster-general in the duke of Wellington's ministry. He voted with his leader on Catholic emancipation, but against theReform Bill in the House of Lords on 7 October 1831, and again when the second reading was carried on 13 April 1832. He also voted for Lord Lyndhurst's motion to postpone the disfranchisement clauses. In autumn 1841 he resigned the lord lieutenancy of Huntingdonshire, which he had held since 1793, owing to failing health, which had never recovered from the accident of 1820. He died from fever at Rome on 18 March 1843.
G. Le G. Norgate, rev. Lynn Milne