William Ellery was born in Newport, Rhode Island on December 2, 1727, the second son of william Ellery, Sr. and Elizabeth Almy, a descendant of Thomas Cornell. He received his early education from his father, a merchant and Harvard College graduate. He graduated from Harvard College in 1747, where he excelled in Greek and Latin. He then returned to Newport where he worked first as a merchant, next as a customs collector, and then as Clerk of the Rhode Island General Assembly. He started practicing law in 1770 at the age of 43 and became active in the Rhode Island Sons of Liberty.
Ellery married Ann Remington of Cambridge, Massachusetts in 1750. She was the daughter of Judge Jonathan Remington (1677-1745). She died in 1764 in Cambridge and was buried there, and he married Abigail Cary in 1767. He had 19 children, and his descendants include Ellery Channing, Washington Allston, william Ellery Channing, Richard Henry Dana, Sr., Edie Sedgwick, and Kyra Sedgwick. Francis Dana married his daughter Elizabeth.
Statesman Samuel Ward died in 1776, and Ellery replaced him in the Continental Congress. He became a signer of the Articles of Confederation and one of the 56 signers of the Declaration of Independence in 1776. The size of his signature on the Declaration is second only to John Hancock's famous signature.
Ellery also served as a judge on the Supreme Court of Rhode Island, and he had become an abolitionist by 1785. He was the first customs collector of the port of Newport under the Constitution, serving there until his death, and he worshipped at the Second Congregational Church of Newport.