Mercer Island is a city in King County, Washington, United States located on an island of the same name in the southern portion of Lake Washington. Just over five miles long and two miles wide, Mercer Island lies in Lake Washington east of the City of Seattle and west of the City of Bellevue. In 1860, a government land survey names Mercer’s Island for the first time. Later shortened to Mercer Island, the island is named for Thomas Mercer (1813-1898), an early pioneer who suggested the names of Lake Washington and Lake Union.