Located on U.S. Highway 1, at approximately mile markers 57--60, below the Conch Keys.

DeBrahm chart (1772) shows Ellis Island. Unnamed on Blunt chart (1846). Gerdes, in his "Reconnaissance of the Florida-Reefs and all the Keys" (1849) states: "The string of islands from Tom's Island (Tom's Harbor Keys) to the West, except the last, larger one, is called Grassy Islands." Grassy Key on Charles Smith's 1873 survey map.

A field report written for the U.S. Coast & Geodetic Survey in 1935-36 states: "Local information is that the key was named after an old settler and not because it was partially covered with grass." Prior to the completion of the highway, the automobile ferry from Lower Matecumbe docked here. The original settlement was called Crainlyn, and was named for the Crain family.