He commanded a body of militia on Dominica when the island was captured by the French in September 1778, and his small garrison, outnumbered, surrendered on terms.

He was sent to North America as commanding engineer in the province of Quebec from 1785 to 1791, served under the Duke of York in Holland in 1793, and in 1794 went back to the Canadas, where he remained till 1804, when he went home to England.

[1] Towards the end of 1775 Mann was posted to Dominica in the West Indies and while there was promoted engineer extraordinary and captain lieutenant on 2 March 1777.

He was only detained for a few months as a prisoner of war, and on 19 August 1779 he was appointed to the engineer staff of Great Britain, and reported on the defences of the east coast of England.

In 1781 he was selected by Lord Amherst and Sir Charles Frederick to accompany Colonel Braham, the chief engineer, on a tour of survey of the north-east coast of England, to consider what defences were desirable, as seven corporations had submitted petitions on the subject.

[3] Promoted captain on 16 September 1785, he was employed in every part of the country in both civil and military duties, erecting fortifications, improving ports, and laying out townships, such as Toronto and Sorel.

[1] In 1788 the governor, Guy Carleton, Lord Dorchester, had him make an extensive examination of military posts, harbours and navigable waterways from Kingston to St. Marys River, Sault Ste.

He went to the Netherlands in 1792,[3] and, joining the British army under Prince Frederick, Duke of York in June 1793, took part in the Flanders campaign.

[1] On his return to England in April 1794 Mann was briefly employed under the master-general of the ordnance in London, before being sent back to Lower Canada, as commanding engineer,[1] to prepare defences at Quebec, since invasion from the United States then seemed a possibility.

On 23 July 1811 he succeeded General Robert Morse as inspector-general of fortifications, an office he held until his death,[1] and in that capacity he continued to write on Canadian defences, such as the construction of the Citadelle of Quebec.