The neighborhood emerged between 1799 and 1810 because the federal government needed somewhere for its temporary employees to sleep. The Capitol was rising at the western edge of the hill; the Washington Navy Yard, established in 1799, was operating along the Anacostia just south. Members of Congress did not yet bring their families to Washington for the sessions. They came alone, rented rooms in boarding houses within walking distance of the Capitol, ate together in shared dining rooms, and went home to their districts when Congress adjourned. The earliest Capitol Hill rowhouses were built to serve them. The Marine Barracks at 8th and I Streets SE, established in 1801, are the oldest active Marine Corps installation in the country and still serve as the residence of the Commandant. Capitol Hill grew the way port-and-government towns always grow, in waves of brick and pressed-brick, each generation adding its own ornament.

Walk a single block on Capitol Hill today and you can read a century of American architectural fashion in the housefronts. Early 19th century manor houses and Federal-style townhouses survive from the boarding-house era. Italianate brackets from the 1850s sit between simple frame dwellings older than the Civil War. The big build-out came after the Civil War, when pressed-brick rowhouses with bay fronts, terra cotta details, and exuberant decorative iron work climbed up the hill in long unified blocks. These late 19th century buildings mix Richardsonian Romanesque arches, Queen Anne towers, and Eastlake spindlework with the kind of unselfconscious abundance that Victorian builders considered good taste. Capitol Hill is one of the largest historic districts in Washington, with around 35,000 residents packed into just under two square miles, making it one of the city's densest neighborhoods.

When people in news broadcasts say Capitol Hill, they almost always mean Congress. The U.S. Capitol sits at the western edge of the neighborhood on its namesake hill, with the Library of Congress, the Supreme Court, and the Senate and House office buildings clustered around it. The Folger Shakespeare Library on East Capitol Street holds the world's largest collection of Shakespeare materials and the third-largest collection of English books printed before 1641. Just south, beside the Anacostia, the Washington Navy Yard predates everything else federal in the neighborhood and still operates today. Congressional Cemetery, founded in 1807, holds the graves of John Philip Sousa, J. Edgar Hoover, and dozens of senators and representatives who died in office in the era before refrigerated rail. All of it is part of what the federal government formally calls the Capitol Complex. The metonym for Congress is also a real neighborhood with real geography.

The center of Capitol Hill's daily life, the part that has nothing to do with Congress, is Eastern Market on 7th Street SE. The market was built in 1873 to a design by Adolf Cluss, the same German immigrant architect responsible for Calvary Baptist Church and much of the Smithsonian's nineteenth-century architecture. Vendors still sell fresh meat and produce in indoor stalls, and an outdoor farmer's market and flea market fill the streets every weekend. A devastating fire gutted the main building on April 30, 2007. The neighborhood rebuilt it. The market reopened on June 26, 2009, two years later, with the original masonry walls preserved and the interior carefully reconstructed. Just south on 8th Street SE, Barracks Row, named for its proximity to the Marine Barracks, is one of the oldest continuously operating commercial corridors in Washington. Fragers Hardware, on Pennsylvania Avenue for nearly a century, also burned and also rebuilt on the same location.

Capitol Hill is full of small squares laid out in the L'Enfant Plan and the streets that converge on them. Lincoln Park, east of the Capitol on East Capitol Street, holds Thomas Ball's 1876 Emancipation Memorial, which depicts Abraham Lincoln standing over a kneeling freedman, a pose that has been controversial since the day it was unveiled. The same park holds Robert Berks's statue of Mary McLeod Bethune, the educator and civil rights leader, the first monument to a Black American or to a woman on federal land in the District. Stanton Park honors Edwin Stanton, Lincoln's Secretary of War. Folger Park, Seward Square, Marion Park (called Turtle Park by anyone with kids, for the cement turtle in its playground), and Garfield Park trace a network of green spaces that the rowhouses face. Notable past residents include John Philip Sousa, Bernie Sanders, J. Edgar Hoover, and Albert Gallatin, four Americans whose biographies overlap nowhere except on these blocks.