He was on 20 September 1740 promoted to be lieutenant of the Defiance, one of the ships going out to the West Indies with Sir Chaloner Ogle, and in her was present at the unsuccessful attack on Cartagena in March–April 1741.
Montagu ordered the sentry to fire at her, and the boat then came alongside with a negro in her, shot through the calf of the leg, who, through the incompetence of the surgeon of the Mercury, bled to death.
The surgeon was dismissed the service by sentence of court-martial, but Commodore Knowles, apparently believing that Montagu was to blame for the man's death, suspended him from his command, and sent him under arrest on board the Eltham for a passage to England.
Heavy damages, it is said, would have been awarded, but his counsel only demanded a nominal penalty often guineas and the costs of the suit.
Meantime he was appointed to the Prince Edward on 20 August 1745, and in July 1746 to the Bristol of 50 guns, one of the ships in the squadron under George Anson in the following spring.
Boscawen, who had charge of a large convoy of East Indiamen, wrote to Anson on the 21st that he had been obliged to confine Montagu, at the desire of the governor, for threatening the life of one of the captains of the Indiaman (Barrow, p. 160; Addit.
But though Charnock, who tells them, presumably received them from William Locker and from Forbes, it is not altogether improbable that they had been told, before their time, of men of the older navy.
Montagu married Charlotte, daughter of Francis Nailour of Offord Darcy in Huntingdonshire, but died without issue (Collins, Peerage, iii.