Mangles was the son of John Mangles (1760–1837), brother of James Mangles (1762–1838), the Member of Parliament. He entered the navy in March 1800, on board the frigate , with Captain Ross Donnelly, whom in 1801 he followed to . After active service on the coast of France, at the reduction of the Cape of Good Hope, and at the British invasions of the Río de la Plata, he was, on 24 September 1806, promoted to be lieutenant of , in which, in February 1809, he was present at the reduction of Martinique.