California Code, Government Code - GOV § 23121

The boundaries of Marin County are as follows:

Beginning in the Pacific Ocean at the southwestern corner of Sonoma; thence southeasterly along the southern line of Sonoma, to the mouth of Petaluma Creek; thence to the common corner of Marin, Sonoma, Contra Costa, and Solano, in San Pablo Bay; thence southerly along the western boundary of Contra Costa, in the Bay of San Pablo, to the middle of the Straits of San Pablo; thence southerly, in a direct line, to Invincible Rock, in the Bay of San Francisco, near the entrance of the Straits of San Pablo; thence, in a direct line, to the northwestern point of Red Rock; thence southerly to the extreme southerly point of Angel Island; thence southwesterly to the extreme end of Point Cavallo at low-water mark; thence on the line of low-water mark along the northern shore of the bay to Point Bonita, and three miles into the Pacific Ocean, to the northwestern corner of San Francisco; thence northwesterly by ocean shore to the place of beginning.