[1][2] The island is named after August Belmont Jr., who financed the construction of the subway tunnel, and in 1977 was dedicated to the memory of U Thant, the former Secretary-General of the United Nations.

The islet contains a lighted beacon marking the southern end of Roosevelt Island Reef and is the home to a small colony of double-crested cormorants.

[3][4] It lies midway between the United Nations Headquarters at 42nd Street, in Manhattan to the west, and Gantry Plaza State Park in Long Island City, within Queens to the east.

The tunnels, which pass directly beneath the island, are now part of the New York City Subway system and used by the IRT Flushing Line (7 and <7>​ trains).

[23][24] Permission to construct two shafts on Man-o'-War Reef and create a temporary island to support the staging of equipment was obtained from Robert Shaw Oliver of the War Department on June 28, 1905.

[22]: 229  Permission to sink a shaft in the East River had originally been requested from the Corporation Counsel, which referred the matter to the Commissioner of Docks and Ferries, but no records of grants of land under water from the state to the city were found in the vicinity of the proposed shaft site, which prompted the Corporation Counsel to advise the Attorney General of New York of the matter.

[21][28][31] In 1909, Commissioner of Public Charities Robert W. Hebberd proposed connecting the southern tip of Blackwell's Island to Man o' War Reef using landfill to create an additional 16 acres (6.5 ha) of park space designed by architect Raymond F. Almirall.

The tunnel was purchased by the city, which reconstructed it beginning in 1914 to accommodate subway cars; this work also included filling in the shaft on Man o' War Reef.

[34][35] On July 7, 1926, the submarine S-51 ran aground at Man-o'-War Reef while it was being towed down the East River to the Brooklyn Navy Yard.

The submarine had been raised from the bottom of the ocean earlier in the week after it had sunk off Block Island and was being taken to the Brooklyn Navy Yard for salvaging.

[36] In the late 1930s, the U.S. Waterways Experiment Station conducted a study of tidal currents in the East River to improve alignments through Hell Gate and flow conditions in the vicinity of Belmont Island, the latter of which involved a tendency of northwesterly river currents during the flood tide to direct ship traffic towards the Manhattan shore.

He later presented his idea to a group of United Nations ambassadors during a luncheon held at the Hotel Pierre, but no funds were available to construct the project, which was then estimated to cost $10 million.

[46][47][48][49] On August 25, 1972, the island was declared "Soviet Jewry Freedom Island" and symbolically occupied for 2+1⁄2 hours by six activists led by Manhattan and Bronx Borough Presidents Percy Sutton and Robert Abrams to protest a United Nations speech by Leonid Brezhnev and the imposition of the diploma tax as a barrier to emigration from the Soviet Union.

[50][51] Other members of the group that landed on the island included Rabbi Gilbert Klaperman, chairman of the Greater New York Conference on Soviet Jewry, and Sister Rose Thering of Seton Hall University.

The group rented a tugboat for reporters and camera crews to cover the event and carried a 20 by 6 foot (6.1 m × 1.8 m) banner displaying the new name of the island.

An entry by Caples Jefferson Architects proposed a granite obelisk on U Thant Island that would gradually disintegrate, leaving only the time capsule by the end of the 30th century.

[64] The reefs in the waters surrounding the U Thant Island make it a popular spot for boats fishing for striped bass and bluefish.