In May 1844, Markham was introduced by his aunt, the Countess of Mansfield, to Rear Admiral Sir George Seymour, a Lord of the Admiralty. The boy made a favourable impression on the admiral, and the meeting led to the offer of a cadetship in the Royal Navy. Accordingly, on 28 June 1844, Markham travelled to Portsmouth to join Seymour's flagship, . "Collingwood" was being fitted out for an extended voyage to the Pacific Ocean where Seymour was to assume command of the Pacific station. This tour of duty lasted for almost four years. The ship reached the Chilean port of Valparaíso, the headquarters of the Pacific station on 15 December 1844, after a cruise that incorporated visits to Rio de Janeiro and the Falkland Islands, and a stormy passage in the Southern Ocean. Markham's social connections assured him of a relatively comfortable time; he was frequently invited to dine with the admiral, whose wife and daughters were on board.