The Custom House Maritime Museum occupies a Greek Revival building that has served as New London's custom house since it was opened in 1835. Exhibits and programs illustrate aspects of New London's multi-faceted maritime history, among them the Amistad Affair (the slave ship captured in 1839 and first brought to New London); area lighthouses; the whaling industry; the War of 1812; yachting; naval submarine history; and customs collection.

The Society and its Museum sponsor a variety of programs designed to inform the public about the maritime significance of New London, neighboring coastal towns, and Long Island Sound. Programming includes exhibits, lectures, informal talks, tours, and school outreach programs. The collections of the Frank L. McGuire Maritime Library document the story of New London's maritime past and present through books, documents, maps, photographs, prints and manuscripts. Through a cooperative program with the federal government, the Society has become the custodian of three nearby lighthouses: New London Harbor Light, New London Ledge Light, and Race Rock Light. The U.S. Coast Guard maintains the beacons while the Society is responsible for the structures. Boat tours and educational lighthouse events are now included among the Society's public outreach programs.

The New London Maritime Society was formed in 1983 by a group of concerned citizens led by the late Lucille Showalter whose goal was to preserve the custom house after the federal government decided it did not need an entire building for its customs work. The two-story Greek Revival landmark was designed by Robert Mills, a federal architect mentored by Thomas Jefferson. A customs office is still maintained, making this the oldest operating custom house in the nation. Among Mills's most notable designs are the Washington Monument, the U. S. Treasury and the Old Patent Office, now a branch of the Smithsonian Institution. The Maritime Society's McGuire Library preserves Mills's portfolio of elegant architectural renderings of the custom house as well as a large cache of letters written by Customs Collector Ingoldsby Crawford when the building was still new. In 2010 the Museum was designated the first Connecticut site on the "Underground Railway to Freedom Trail" being implemented by the National Park Service.