The Custom House is currently operated by Preservation Long Island and used to educate the public about the history of Sag Harbor and the Dering family.

Around 1761, the first wharf was built to support the whaling industry, serving as a catalyst for Sag Harbor's growing economy.

Following the ratification of the United States Constitution, Sag Harbor became a Federal Port of Entry in 1789, only second in importance to New York City.

In addition to being a signer of the Continental Association (1774), Thomas Dering served as the inspector of trade and navigation at the Port of New London before the Revolutionary War.

In addition to serving as custom master, Dering was also made inspector of the revenue and collector of Sag Harbor in 1791, and postmaster in 1794.

With these jobs, Dering was responsible for weighing and gauging cargoes, hearing oaths of registry and citizenship, and overseeing statements regarding truthfulness and reliability of mail delivery.

[2] The first floor of the Custom house is organized around a center stair hall located opposite the front door.

Connected to the custom room is another lean-to structure with an exterior entrance that originally functioned as the post office, making the entire south side of the house reserved for Dering's work.

[2] Interest in the preservation of the Custom House emerged in 1944 when it caught the attention of Henry Triglar Weeks, founder of the Old Sagg-Harbour Committee, an organization devoted to finding, saving, and restoring old buildings in the Sag Harbor area.

As the Custom House faced demolition, Weeks worked to save it, recruiting architects like Talbot Hamlin to express their support for its preservation.

Finally in 1948, Weeks facilitated the gifting of the Custom House from St. Andrews Church to former Governor Charles Edison's Argonaut Enterprises.

In 1955, the Old Sagg Harbour Committee became a non-profit museum and assumed ownership of the Custom House from Argonaut Enterprises, Inc.

Historian Loring McMillen played an important role in this process, recommending the house be interpreted to the period of 1794 to 1810.

In addition to giving tours, Preservation Long Island also holds on-site educational programs that explore maritime trade and the role of custom houses in the early United States.