William Thornton (May 20, 1759 – March 28, 1828) was an American physician, inventor, painter and architect who designed the United States Capitol. He also served as the first Architect of the Capitol and first Superintendent of the United States Patent Office.

From an early age, William Thornton displayed interest and discernible talent in "the arts of design," to employ an 18th-century term that is particularly useful in assessing his career. Thornton was born on Jost Van Dyke in the British Virgin Islands, West Indies, in a Quaker community. where he was heir to sugar plantations. He was sent to England at age five to be educated. Thornton was brought up strictly by his father's relations, Quakers and merchants, in and near the ancient castle town of Lancaster, in northern Lancashire, England. There was never any question of his pursuing the fine arts professionally—he was to be trained for a useful life, according to the Quaker ways. Thus, despite the fact that he had a sizeable income, young Thornton was apprenticed for a term of four years (1777–1781), to a practical physician and apothecary in Ulverston, Lancashire (now Cumbria).

The earliest of Thornton's known writings, a journal he began during his apprenticeship, records almost as many entries for drawing and sketching as notes on medical treatments and nostrums. His subjects were most often flora and fauna, but he also did portraits, landscapes, historical scenes, and studies of machinery, such as the Franklin stove, and managed to construct a camera obscura. This pattern continued when he enrolled as a medical student in the University of Edinburgh in 1781. He interned at St Bartholomew's Hospital. The architecture of Edinburgh, especially that of the New Town that was being built, surely exerted considerable influence. More direct evidence of his interest in architecture is found in the landscapes and sketches of castles he drew while travelling about Scotland, notably in the Highlands, during these years.

In 1783, Thornton went to London to continue his medical studies; characteristically, he also found time to attend lectures at the Royal Academy. The following year he was off to the Continent, carrying a letter of introduction to Benjamin Franklin, (1706–1790), written by his mentor and distant cousin John Coakley Lettsome, (1744–1815). In the summer of 1784, he explored the Highlands with French geologist Barthélemy Faujas de Saint-Fond. He received his medical degree in 1784 at the University of Aberdeen in Scotland.

Thornton then spent time in Paris, before returning to Tortola in 1786. There, he saw his mother for the first time since boyhood, where he came face to face with the source of his income—half interest in a sugar plantation and ownership of some 70 slaves, the possession of whom had begun to trouble him.

Eager to achieve fame (and undoubtedly some expiation) in the cause of anti-slavery, he emigrated to the United States of America in the fall of 1786, moving to Philadelphia. His unsuccessful efforts to lead a contingent of free black Americans to join the small British settlement of London blacks at the mouth of the Sierra Leone River in West Africa were looked on favorably by Philadelphia's Quaker establishment. Some leaders of the new republic—notably James Madison, with whom he lodged at Mrs. Mary House's prominent boarding establishment in 1787 and 1788—were cognizant of Thornton's abolitionist activities. However, after moving to the City of Washington, he took advantage of slavery. According to a diary his wife kept in 1800, he frequently shopped for slaves and bought and hired them. In 1788, he became an American citizen. Thornton married Anna Maria Brodeau, daughter of a schoolteacher, in 1790.

In 1789, after briefly practicing medicine and pursuing an interest in steamboats, Thornton submitted a design to the architectural competition for the Library Company of Philadelphia's new hall. His design won but was somewhat departed from during actual construction. Library Hall was described as the first building in the "modern [classical] style" to be erected in the new nation's leading city.

During his visit to Tortola between October 1790 and October 1792, Thornton learned of the design competitions for the U.S. Capitol and the "President's House" to be erected in the new Federal City on the banks of the Potomac. Because a design for the Capitol had not been chosen, he was allowed to compete upon his return to Philadelphia. Between July and November 1792, the Washington administration examined closely the designs submitted by the French émigré architect Etienne Sulpice Hallet, (1755-1825), and Judge George Turner. Hallet and Turner had been summoned to the Federal City in August 1792 to present their ideas to the "Commissioners of the District of Columbia" and local landholders. Both were then encouraged to submit revisions of their designs to accommodate new conditions and requirements. At the beginning of November, Turner's new designs were rejected.