He was born at Gillingham, near Chatham, England. After losing his father at the age of 12, he was apprenticed to shipyard owner Master Nicholas Diggins at Limehouse for the seafaring life. He spent the next 12 years learning shipbuilding, astronomy and navigation afterwards entering the British navy. After serving in the Royal Navy under Sir Francis Drake, Adams became a pilot for the company Barbary Merchants. During this service, he took part in an expedition to the Arctic that lasted about two years in search of a Northeast Passage along the coast of Siberia to the Far East.
He set sail from Rotterdam in June 1598 on the Hoop and joined up with the rest of the fleet (Liefde, Geloof, Trouw and Blijde Boodschap) on June 24.
The vessels, boats ranging from 75 to 250 tons and crowded with men, were driven to the coast of Guinea, where the adventurers attacked the island of Annabon for supplies, and finally reached the straits of Magellan. Scattered by stress of weather the following spring the Liefde with Adams on board, and the Hoop met at length off the coast of Chile, where the captains of both vessels lost their lives in an encounter with the Indians.
Adams changed ships to the Liefde (originally Erasmus because of the wooden figurehead of Erasmus on her bow) and waited for the other ships at Santa Maria Island. Only the Hoop arrived. It was late November 1599 when the two ships sailed westwardly for Japan. In fear of the Spaniards, the remaining crews determined to sail across the Pacific. On this voyage a typhoon claimed the Hoop in late February 1600, but in April 1600 the Liefde with a crew of sick and dying men, was brought to anchor off the island of Kyushu, Japan. When the Liefde made landfall April 19, 1600, off Bungo (present-day Usaki City, Oita Prefecture), only nine of the remaining 24 crew members could even stand. Allegations by Portuguese priests that Adams' ship was a pirate vessel led to seizure, and the sickly crew was imprisoned at Osaka Castle on orders by the Shogun Tokugawa Ieyasu. Here Adams was questioned by lyeyasu, the guardian of the young son of Taiko Sama, the ruler, who had just died. Adams knowledge of ships and shipbuilding, and his nautical smattering of mathematics, appealed to the shogun. In 1604, Ieyasu ordered Adams to build a western-style sailing ship at Ito, on the east coast of the Izu Peninsula. An 80-ton vessel was completed and the Shogun ordered a larger ship, 120 tons, to be built the following year. The Shogun took a liking to Adams, and made him a revered diplomatic and trade adviser and bestowing great privileges upon him.
Adams had a wife and children in England, but Ieyasu had forbidden the Englishman to leave Japan. He was presented with two swords representing the authority of a Samurai. The Shogun decreed that William Adams the pilot was dead and that Miura Anjin, a samurai, was born. This made Will's wife in England, in effect, a widow, and "freed" Adams to serve him on a permanent basis. He was granted a fief in Hemi within the boundaries of present-day Yokosuka City, a handsome salary. This gave him the means to marry Oyuki, the daughter of Magome Kageyu, a noble samurai and official of Edo Castle, which stood in present day Tokyo. Anjin and Oyuki had a son called Joseph, and a daughter, Susanna. The Anjin, however, found it hard to rest his feet and was constantly on the road. Initially, it was in the vain attempt to organize an expedition in search of the Arctic passage that had eluded him previously.
In 1611 news came to him of an English settlement in Bantam, and he wrote asking for help. In 1613 Captain John Saris arrived at Hirado in the ship Clove with the object of establishing a trading factory for the British East India Company, and after obtaining the necessary concessions from the shogun, Adams postponed his voyage home (permission for which had now been given him) in order to take a leading part, under Richard Cocks, in the organization of this new English settlement. The latter part of his life was spent in the service of the English trading company, for whom he undertook a number of voyages to Siam in 1616, and Cochin China in 1617 and 1618.
He died at Hirado, north of Nagasaki on May 16, 1620 at the age of 56, some three years before the dissolution of the English factory. His Japanese title was Anjin Sama, and his memory was preserved in the naming of a street in Yedo, Anjin Cho (Pilot Street), and by an annual celebration on June 15 in his honour.