John Montagu (21 August 1797 – 4 November 1853) was private secretary to the colonial secretary of Van Diemen's Land (Tasmania) 1834–1842 and later of Cape Colony.
Montagu was born in India, the second son of Lieutenant-Colonel Edward Montagu, and his wife Barbara, née Fleetwood. Edward Montagu was great-great-grandson of Lord James Montagu (d. 1665), who was younger son of Henry Montagu, 1st Earl of Manchester. Edward died of wounds in India in 1799. John Montagu sent to England to be educated; he was taught at Cheam in Surrey, Parson's Green in Knightsbridge, and by a private tutor. In February 1814 Montagu was commissioned Ensign in the 52nd (Oxfordshire) Regiment of Foot.
Montagu fought at Waterloo, was promoted Lieutenant in November 1815, and Captain in the 64th Foot in November 1822. In 1823, having transferred to the 81st Foot and then the 40th Foot, he went to Van Diemen's Land in 1824 with Governor Arthur and became Arthur's private secretary. In 1826 he was made clerk of the Executive and Legislative Councils, but in 1829 was recalled to England to take up his military duties. In 1830 he resigned from the army and was re-appointed clerk of the councils at Hobart.
In 1832 Montagu acted as colonial treasurer, and in 1834 was appointed colonial secretary. Montagu was still in this position when Sir John Franklin became governor in 1836, and for five years the two men worked in harmony. Montagu devoted time to the question of convict discipline, and in 1841 prepared the necessary instructions for a probation system which was then established. In October 1841 a strong difference of opinion arose with governor Franklin over the reinstatement by Franklin of a surgeon who had been dismissed after being charged with culpable negligence. Franklin reinstated him because he thought that further evidence showed the penalty to have been unjust, Montagu declared that the reinstatement would degrade the colonial secretary's office, and that if Franklin persisted in his determination he must not expect the same assistance from the colonial secretary that had been hitherto given. Franklin would not be intimidated and friction continued for some time. On 17 January 1842 in writing to Franklin, Montagu said,