Thomas Jefferson while vice president was a prime mover in the creation of the library, and he supported it strongly during his presidency. In 1814, when much of the collection was destroyed by fire, Jefferson offered his own fine library to the Congress. This formed the basis of the collection until 1851, when fire destroyed some 35,000 volumes. The growth of the library progressed slowly thereafter until the passage of the Copyright Act of 1870, which required the deposit in the library of all copyright material. The acquisition in 1866 of the Smithsonian Institution's collection of 44,000 volumes and the purchase of the Peter Force collection of Americana (60,000 volumes; 1867) and the Joseph M. Toner American and Medical Library (24,000 volumes; 1892) made it one of the world's great libraries.

Originally primarily intended to serve the legislative branch of the government, it is now open to the public as a reference library and sends out many books through an interlibrary loan system. It has African and Middle Eastern, Asian, European, and Hispanic divisions; a law library; and excellent collections of manuscripts, periodicals, monographs and serials, incunabula, geography and maps, rare books, prints and photographs, motion pictures, music and recordings, sheet music, science and technology, visual materials, microforms, and computer files, representing materials in more than 450 languages.

The Library of Congress contains more than 138 million items, including about 21 million books, 5 million maps, and 61 million manuscripts. Its Online Catalog provides a database of some 12 million items from its collections. The library sells duplicate catalog entries to smaller libraries for the books it adds to its collections. It provides other vital services to libraries through its many bibliographic functions (among them maintaining the National Union Catalog of the holdings of 700 large libraries in the United States and running the National Library Service for the Blind and Physically Handicapped) and its Copyright Office. The library's Poetry and Literature Center (est. 1936) is the home of the U.S. poet laureate. The National Audio-Visual Conservation Center, opened in Culpeper, Va., in 2007, is the home of the library's large film and recording collection. Mainly supported by congressional appropriations, the library also has income from gifts by foundations and individuals, administered by the Library of Congress Trust Fund Board.