The Library of Congress card catalog is often thought to be the "original" catalog of the Library of Congress's collections, but that is not the case. For the first one-hundred years of the Library's existence (1800-1900) the Library created and published print catalogs to its collections. These catalogs varied in accuracy, quality, and comprehensiveness, but were the best tools for determining what works were held by the Library. When the Library assumed copyright registration and deposit responsibilities in 1870, the huge increase in the number of books and other items acquired by the Library rendered the most recently published catalogs quickly obsolete. The influx of books quickly created a space crisis as the Library ran out of shelf space in the U.S. Capitol, the Library's 19th century home. Attempts to update the print catalog were sidelined until the Library's space issues could be addressed.

When the Library of Congress Building (now known as the Thomas Jefferson Building) opened in 1897, the Library for the first time had its own, copious space with enough shelving for all of its collections, and efforts could be turned to the monumental task of how best to classify and catalog the collections.

While plans to reclassify the Library's collections were under discussion, the Library's Main Reading Room provided access to an author catalog (later referred to as the "old official catalog") at its Center Desk. One of the best short descriptions of this catalog appears on page 9 of the 1907 pamphlet The Library of Congress and Its Work:

This catalog is still in use, being located in the Reading Room immediately back of the New Card Catalog .

It consists of entries clipped from the catalog of 1864, the accession lists issued between 1864 and 1876, and the section of the author catalog printed in 1878 – 1880 ( A - Drei ). These entries are mounted on cards, 4 1/2 by 7 inches. Entries for later accessions, or for books which did not appear in the printed catalogs mentioned, were supplied in ms., with the exception of a few which were clipped from the Catalog of title entries issued by the Copyright Office and mounted on cards.

This catalog was kept up until the end of 1899. It is now being replaced by the New Card Catalog .

The development of the New Card Catalog was begun in July 1898. In early 1900 the card catalog became available in the Main Reading Room, and was usually referred to as the Public Catalog. As noted in the Library's 1901 Annual Report External, "the old author catalogue has been kept behind this desk, but it is now to be brought out into the space accessible to readers. The new card catalogue by author and subject, which will gradually absorb the old, is already accessible outside of the desk." Card catalog cases in which the catalog was housed were added to the Main Reading Room in 1902.

The initial version of the Public Catalog included entries for the 1864 print catalog and its various supplements through 1876, totaling about 160,000 titles. In January 1901 the Library began printing catalog cards for all newly cataloged or re-cataloged books. The author catalog was discontinued shortly thereafter.

The card catalog grew during the proceeding decades, expanding into what is now the Reference Assistance Room. In 1970, the card catalog further expanded into was is know the Microform and Electronic Resources Center (LJ 139), displacing the National Union Catalog card file that had been located there for approximately forty years.

The card catalog grew to more than 22 million cards in 22,000 drawers before the last card was added in 1980. At its peak, the catalog occupied approximately one-fifth of the Main Reading Room, all of the current Reference Assistance Room (RAR) that leads into it, the entirety of Room LJ 139, and parts of the hallways between the RAR and LJ 139.

When the Main Reading Room closed for renovation on December 9, 1987, the card catalog was relocated temporarily to Deck 7 South in the Adams Building. When the Main Reading Room reopened on June 3, 1991, the card catalog was moved back to the Jefferson Building, but relocated from the spaces it previously occupied in the Main Reading Room to Decks 33 and 37 adjacent to it, where they remain accessible to the public. The original catalog's decorative end panels were preserved and modified for use in the Computer Catalog Center (now the MERC) adjacent to the Main Reading Room. In addition, the catalog unit's interiors were fitted with work surfaces on which Computer Catalog Center terminals and printers were placed.