Mill Rock is located at Hell Gate, formerly an infamously treacherous area for ships to pass.

This fortification was part of a chain of blockhouses that was intended to defend New York Harbor and protect the passage into Long Island Sound against the British Navy.

[3] The United States Army Corps of Engineers started clearing rocks from Hell Gate in the late 19th century.

In 1885, USACE detonated 300,000 lb (136,000 kg) of explosives on adjoining Flood Rock; that island had been the most treacherous impediment to East River shipping.

[6] The herons and egrets are thought to have moved to Mill Rock from nearby North Brother Island.