While he started as an able seaman aboard H.M.S. Eagle, within a month was a master's mate and four years later, became the sailing master of his ship. After two years in the English Channel, he was promoted to master aboard the Pembroke, and in 1758 crossed the Atlantic to participate in the siege of Louisburg and a survey of the St Lawrence River that resulted in the capture of Quebec.
After a transfer to the Northumberland, he spent the next five summer seasons surveying the coasts of Nova Scotia and Newfoundland, producing the first accurate large-scale maps in hydrographic surveys to use precise triangulation and continuing his scientific studies while he wintered in Halifax.
Back in England late in 1762 he married but returned to the Newfoundland survey. He was given his first command (the Grenville) in 1764.
The publication of his charts of Newfoundland waters and an observation of a solar eclipse brought him to the attention of influential figures at the Admiralty and in the Royal Society, which had recommended Alexander Dalrymple to lead an expedition to the South Pacific to observe the transit of Venus.
But Dalrymple was a civilian and the Navy insisted that one of their own men would command the expedition. The Admiralty promoted Cook from master to lieutenant and gave him command of the 368-ton bark Endeavour, a former Whitby collier which sailed from Plymouth on 26 August 1768.
While the purpose of the voyage was to observe the transit of Venus across the Sun Cook had secret instructions to establish whether the long-supposed southern continent, Terra Australia Incognita, actually existed.
The Endeavour rounded Cape Horn, continued westward across the Pacific and arrived at Tahiti on 13 April 1769.