Mercer Island is a city in King County, Washington, located on an island of the same name in the southern portion of Lake Washington. The ZIP code 98040 is unique to Mercer Island. Mercer Island has the fifth highest per-capita income in the state of Washington and is one of the 100 richest ZIP codes in the USA according to the IRS. Mercer Island is Washington state’s most expensive housing market. Mercer Island is an excellent place to live with a large variety of amenities and access to two large cities on either side. The Mercer Island Farmers Market operates most Sundays between June and October. A special version of the market called the Harvest Market occurs on a Sunday in November.