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.

[2] 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.

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).

[3] 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.

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.

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).

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.

[7] 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.

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.

The painter John Trumbull handed in William Thornton's still "unfinished" revised plan of the Capitol building on January 29, 1793, but the President's formal approbation was not recorded until April 2, 1793.

Thornton was inspired by the east front of the Louvre, former royal palace later turned art museum, as well as the Pantheon, famed former Roman temple in Roma and later converted to a Christian church, for the center portion of the design.

[8] After more drawings were prepared, enthusiastic praise of Thornton's design was echoed by Secretary of State Jefferson: "simple, noble, beautiful, excellently distributed."

As a consequence of winning the Capitol competition, Thornton was frequently asked to give ideas for public and residential buildings in the Federal City.