The completed Capitol boasted some of America's most splendid interiors, with statues, marble columns, wall-to-wall carpets, and damask draperies. Oil-burning chandeliers (converted to gas in the 1840s) illuminated legislative rooms furnished with mahogany desks and chairs. The Supreme Court Chamber was more dimly lit, creating a somber atmosphere. Among the most popular rooms was the Library of Congress. Thomas Jefferson sold Congress his private library in 1815, greatly expanding the size and breadth of the collection lost to fire and thereby laying the foundation for today's Library of Congress.

The Capitol covered about 60,000 square feet, with 70 offices and committee rooms warmed by more than 80 fireplaces and dozens of stoves. A restaurant served oysters, roasted meats, and turtle soup. A spring supplied drinking water, and a rooftop cistern collected rainwater to flush the china basins in congressional privies.