Fairfax, California, a small city in Marin County, is believed to be the only city in the United States in which a majority of members of the city’s governing body are members of the Green Party. Fairfax, like all cities in California, uses non-partisan elections. Fairfax has a city council of five members. Recently one of the Greens resigned from the city council, so the remaining Greens, still in the majority, replaced him with a new member who is also a registered Green. See this story on the California Green Party’s web page. Thanks to Michael for the link.