Mount Sinai is a hamlet and census-designated place (CDP) located within the Town of Brookhaven, in Suffolk County, New York, United States.

The area now known as Mount Sinai was originally called Nonowatuck, or "stream that dries up", by the Seatocot family of Native Americans who lived here.

After Gotherson sent representatives who were quickly turned away, local residents started to jokingly refer to the land as "the Old Mans".

During the course of the war, American agents visited local patriots and received information, clothing, supplies and money to support the cause.

In 1780, American Major Benjamin Tallmadge and a detachment of Continental Army dragoons landed at Cedar Beach and traveled south to successfully attack the British at Manor St.

In addition to the farmland that populated Old Mans, the town's trees provided much of the wood for the shipbuilding trade centered in neighboring Port Jefferson.

A commercial center did not develop in Mount Sinai and residents depended on businesses in the neighboring village of Port Jefferson.

In the early 19th century, a Methodist church was built on Shore Road, but this was later sold when the congregation joined with that of Port Jefferson.

Once the Long Island Rail Road reached adjacent Port Jefferson in 1879, and temporarily continued east through Mount Sinai to Wading River from 1895 to 1938, new visitors and residents began appearing from New York City and from elsewhere on Long Island in order to enjoy the hamlet's beaches and bucolic setting.

This consisted of housing and volunteer workers on a 130-acre (53 ha) plot, with the location being chosen due to its close proximity to St. Charles Hospital in adjacent Port Jefferson.

Cedar Beach, which is located on a peninsula that divides the harbor from the Long Island Sound, became a popular recreational destination.

The Chandler Estate, on Mount Sinai Harbor's southern side, was repurposed as vacation cottages and housed Marilyn Monroe and Arthur Miller during their holidays in the 1950s.

Examples of these are the "colony" of homes built off Pipe Stave Hollow Road, both north and south of Route 25A, and the scattered development on Mount Sinai and Chestnut avenues.

During this time a community was mapped but never developed just south of Route 25A and east of Crystal Brook Hollow Road.

The northern part of Mount Sinai Harbor borders an active recreation area that is located on a 2-mile long peninsula.

A marine sanctuary and nature preserve connects the various sections of Cedar Beach with scenic walking trails and serves to protect the remaining sand dunes.

Commonly owned lands set aside in 1664 are being used for public recreation or have been rented for private use by a fishing station, yacht club and boatyard.

The peninsula, which is most directly accessible from the neighboring community of Miller Place, contains a number of historical vacation houses on both sides of the boundary line.

Throughout its history, Mount Sinai Harbor has been utilized for the catching of shellfish, the fishing of snapper and flounder, the hunting of waterfowl, sand milling, and the harvesting of salt hay.

Three major civic battles were waged in the 20th century when various plans to dredge the harbor were proposed and pursued by the Town of Brookhaven.

Two dredging projects were approved, and the present channels and deep basin south of Cedar Beach are the result of that action.