Pierre L'Enfant called it 'a pedestal awaiting a monument.' Jenkins Hill, the small rise at the eastern end of the National Mall, was where the Congress House would stand. In 1792 Thomas Jefferson, having dismissed L'Enfant from the federal city project, announced a design competition for the Capitol. The submissions were uniformly amateurish - American architects in 1792 had little training, and most of the entries looked, in one historian's phrase, like Federal-period barns. A late entry arrived on January 31, 1793, from William Thornton, a Scottish-trained physician who taught himself architecture. Thornton's design borrowed the east front of the Louvre and the dome of the Paris Pantheon. George Washington called it a work of grandeur, simplicity, and beauty. Thornton was appointed the first Architect of the Capitol on April 5, 1793. The cornerstone was laid by Washington in Masonic ceremony on September 18, 1793. The senate wing opened in 1800, when the Sixth Congress moved from Philadelphia. The Capitol has been the home of the U.S. Congress without interruption since.

The 1850s expansion, the largest in the building's history, was carried out partly by enslaved African Americans. The original plan was to import European stoneworkers and carpenters, but recruitment in Europe went poorly. The work fell instead to a mixed labor force of Scottish stonemasons and African Americans, some free and many enslaved. The enslaved workers - hired by the Architect of the Capitol from their owners at fixed daily rates that the owners pocketed - cut the logs, baked the bricks, laid the stones. Some of their names have been recovered from payroll records; most have not. In 2007 Congress commissioned and installed two plaques in Emancipation Hall in the Visitor Center, acknowledging the role enslaved workers played in building the Capitol. The Statue of Freedom on top of the dome - a nineteen-foot bronze figure of a classical goddess in armor - was cast at the foundry of Clark Mills in Bladensburg, Maryland, with most of the foundry work performed by an enslaved master ironworker named Philip Reid. He was emancipated in April 1862 when Lincoln signed the DC Compensated Emancipation Act, partway through the casting, and saw the statue raised to the top of the dome in December 1863. The casting payment ledgers list him by his given name only.

By 1850 the Bulfinch dome of 1822 - low, copper-sheathed, made of timber - had become absurd. The 1850 expansion had doubled the building's length and the small dome looked silly. Thomas U. Walter designed the cast-iron replacement that still stands. It is three times the height of the original, one hundred feet in diameter at the drum, and structured as a double dome - an inner shell visible from the rotunda below, an outer shell visible from outside, with a span of empty space between them through which The Apotheosis of Washington (Brumidi's fresco) is seen by the public. The cast iron weighs nearly nine million pounds. The Janes Fowler Kirtland foundry in New York manufactured the ironwork; the Poole and Hunt ironworks in Baltimore made the thirty-six Corinthian columns around the dome's base. Construction began in 1856 and continued through the Civil War. Lincoln, asked whether the project should be paused, replied: 'If people see the Capitol going on, it is a sign we intend the Union shall go on.' The dome was finished in 1866.

On the afternoon of January 6, 2021, supporters of outgoing President Donald Trump, encouraged by speeches he and others had given at a rally on the Ellipse south of the White House, marched to the Capitol. Inside, both houses of Congress were meeting in joint session to certify the Electoral College results from the November 3 election. Approximately 2,000 rioters breached police lines and entered the building. Members of Congress were evacuated through underground tunnels. Capitol Police Officer Brian Sicknick was assaulted with chemical spray while defending the perimeter, collapsed that evening, and died the following day; the DC medical examiner ruled natural causes - strokes - as the official cause of death. Four other officers who responded that day later died by suicide. Ashli Babbitt, one of the rioters, was shot and killed by Capitol Police while attempting to breach a barricaded doorway near the House chamber. Three other rioters died of medical events during the riot. The Capitol's interior suffered approximately 1.5 million dollars in damage; broken windows, smeared excrement, ransacked offices, defaced bronze busts. Congress resumed the certification that night. The Senate certified the results at 3:40 a.m. on January 7. The building reopened to the public in May 2022 with new fencing and security measures. The breach is the worst attack on the Capitol since the British army burned it in 1814.

Today the Capitol covers approximately four acres, with 540 rooms, sixteen and a half miles of corridors, and 658 windows. The 2008 underground Capitol Visitor Center adds 580,000 square feet beneath the East Plaza, with Emancipation Hall, two orientation theaters, gift shops, and dining areas. The House and Senate chambers occupy the south and north wings respectively. The Old Senate Chamber and Old Supreme Court Chamber on the second floor have been preserved as they appeared in the mid-nineteenth century. National Statuary Hall, formerly the House chamber until 1857, holds two statues from each state. The Crypt below the rotunda - built originally to receive George Washington's body, which never came - holds the empty marble tomb beneath the rotunda floor. The Speaker of the House and the President pro tempore of the Senate have ceremonial offices on the second floor. The Capitol Police force, headquartered in the basement, has roughly 2,300 sworn officers. The dome is repainted every two decades and was last conserved in a 285-million-dollar restoration completed in 2016, the first major dome work since 1960. The building, like the Republic it houses, exists in continuous repair.