William Bligh was born on 9 September 1754, but it is not clear where. It is likely that he was born in Plymouth, Devon, as he was baptised at St Andrew's Church, Plymouth on 4 October 1754, where Bligh's father, Francis (1721–1780), was serving as a customs officer. Bligh's ancestral home of Tinten Manor near St Tudy near Bodmin, Cornwall, is also a possibility. Bligh's mother, Jane Pearce (1713–1768), was a widow (née Balsam) who married Francis at the age of 40. Bligh was signed for the Royal Navy at age seven, at a time when it was common to sign on a "young gentleman" simply to gain, or at least record, the experience at sea required for a commission. In 1770, at age 16, he joined HMS Hunter as an able seaman, the term used because there was no vacancy for a midshipman. He became a midshipman early in the following year. In September 1771, Bligh was transferred to Crescent and remained on the ship for three years.
In 1776, Bligh was selected by Captain James Cook (1728–1779), for the position of sailing master of Resolution and accompanied Cook in July 1776 on Cook's third voyage to the Pacific Ocean, during which Cook was killed. Bligh returned to England at the end of 1780 and was able to give details of Cook's last voyage.
Bligh married Elizabeth Betham, daughter of a customs collector (stationed in Douglas, Isle of Man), on 4 February 1781. The wedding took place at nearby Onchan. A few days later, he was appointed to serve on HMS Belle Poule as master (senior warrant officer responsible for navigation). Soon after this, in August 1781, he fought in the Battle of Dogger Bank under Admiral Parker, which at last won him his commission as a lieutenant. For the next 18 months, he was a lieutenant on various ships. He also fought with Lord Howe at Gibraltar in 1782.
Between 1783 and 1787, Bligh was a captain in the merchant service. Like many lieutenants, he would have found full-pay employment in the Navy; however, commissions were hard to obtain with the fleet largely demobilised at the end of the War with France when allied with the North American rebelling colonies in the War of American Independence (1775–1783). In 1787, Bligh was selected as commander of His Majesty's Armed Transport Bounty. He rose eventually to the rank of vice admiral in the Royal Navy.