 About 1.9% of families and 3.2% 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.Mercer Island uses a Council–manager government and the city council selects the mayor.The City Hall building in the northeast part of the city hosts offices for the City Manager and most local administrative employees, the municipal court, and the police department.