Marin County is one of the original twenty-seven California counties. It was created 18 February 1850.

For most of the 20th century, Marin County was a Republican stronghold in presidential elections, but since 1988, it has been one of the Democrats' major strongholds in California. Only San Francisco County and Alameda County voted more Democratic in the 2008 Presidential election (Wikipedia).

In August 2019, the financial technology company, SmartAsset, listed Marin County as the third richest county in the United States. The computation of "richness" included all sources of income: wages, investments and retirement accounts.

This wealth has allowed Marin County to protect large tracts of open space such as Muir Woods, Point Reyes, Angel Island, China Camp, Mount Tamalpais, Tomales Bay, Olompali, part of the Golden Gate National Recreation Area and part of the Gulf of the Farallones National Marine Sanctuary. The Lagunitas Creek Watershed is home to the largest remaining wild run of coho salmon in Central California.