Mercer Island is a city in King County, Washington, and the name of the island in Lake Washington on which the city sits. The population was 22,699 at the 2010 census. The population was 22,690 at 2012 Estimate from Office of Financial Management. Located in the Seattle Metropolitan Area, it is the most populated island in a lake within the United States. The ZIP code 98040 is unique to Mercer Island. The median income for a household in the city was $151,904, and the median income for a family was $190,830. The average household income in 2004 was $169,841. The per capita income for the city was $123,799. About 0.9% of families and 0.2% of the population were below the poverty line, including 3.4% of those under age 18 and 2.3% of those age 65 or over. As of 2011, the 98040 ZIP code had a median house sale price of $1,001,405, placing it at #209 in a list of the most expensive ZIP codes in America, including a sale of a property for $26.9 million.