When was Marin County founded?

Marin County was founded in 1850, formed from Sonoma County, and named after Chief Marin.

What is the county seat of Marin County?

The county seat of Marin County is San Rafael.

Is there an online parcel map for Marin County?

Yes. Marin County publishes a parcel map of 96,239 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.

Has Marin County's boundary or name changed?

Marin County was created in 1850 from Sonoma County. Since then the Atlas of Historical County Boundaries records 7 later changes, chiefly involving Sonoma County, San Francisco County and Contra Costa County.

Is my lot in a flood zone?

We do not have flood maps for Marin County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

County · code 06041

Marin CountyCalifornia

Marin County, California, is located north of San Francisco Bay and has its county seat in San Rafael. The county was created on February 18, 1850, from land that had not previously been part of any county, and was named after Chief Marin.

Founded1850
County seatSan Rafael
Named forChief Marin
Land area520 sq mi + 308 sq mi water
Population262,321 2020 Census
Parcels mapped96,239 checked 16 October 2026

Open the free map Click any parcel in Marin County: acreage, parcel id, address and who publishes the map. No sign-in.

Updated 19 October 2026

Census outline · seat marked

History

Formed in 1850 from Sonoma County

An old deed may name a county that no longer exists, or a boundary that has since moved.

When Marin County was created on February 18, 1850, it was initially attached to Sonoma County for judicial purposes, but became fully organized and independent just two months later on April 22, 1850. The county experienced several boundary changes in its early years: it gained and lost territory to Sonoma County multiple times between 1855 and 1861, and also exchanged Angel Island in San Francisco Bay with San Francisco County in 1854 and 1857 before regaining it in 1860. In 1868, Marin County lost the Brothers Islands to Contra Costa County.

Boundary changes from the Newberry Library, Atlas of Historical County Boundaries; the 5 that matter most are shown first.

1850
Marin County created Marin created from non-county area; Marin not fully organized, attached to Sonoma "for judicial purposes."
from Sonoma
18 February 1850 · Calif. Stats. 1849, 1st sess., ch. 15, sec. 11/p. 60
1854
Gained territory Marin gained "Island of Los Angeles" (Angel Island) in San Francisco Bay from San Francisco.
from San Francisco
15 May 1854 · Calif. Stats. 1854, 5th sess., ch. 67/p. 121
1855
Lost territory Marin lost to Sonoma.
to Sonoma
27 April 1855 · Calif. Stats. 1855, 6th sess., ch. 125/p. 150
1857
Lost territory Marin lost Angel Island in San Francisco Bay to San Francisco.
to San Francisco
18 April 1857 · Calif. Stats. 1857, 8th sess., ch. 190/pp. 209-210
1868
Lost territory Marin lost the Brothers Islands to Contra Costa.
to Contra Costa
26 March 1868 · Calif. Stats. 1867, 17th sess., ch. 305/p. 347

Show all 9 changes (4 more, mostly small adjustments)

1850
Marin County created Marin created from non-county area; Marin not fully organized, attached to Sonoma "for judicial purposes."
from Sonoma
18 February 1850 · Calif. Stats. 1849, 1st sess., ch. 15, sec. 11/p. 60
1850
Changed Marin fully organized, detached from Sonoma.
Sonoma
22 April 1850 · Calif. Stats. 1849, 1st sess., ch. 131/p. 406
1851
Boundary clarified Marin boundaries redefined (no change).
no change
25 April 1851 · Calif. Stats. 1851, 2d sess., ch. 14, sec. 23/pp. 177-178
1854
Gained territory Marin gained "Island of Los Angeles" (Angel Island) in San Francisco Bay from San Francisco.
from San Francisco
15 May 1854 · Calif. Stats. 1854, 5th sess., ch. 67/p. 121
1855
Lost territory Marin lost to Sonoma.
to Sonoma
27 April 1855 · Calif. Stats. 1855, 6th sess., ch. 125/p. 150
1857
Lost territory Marin lost Angel Island in San Francisco Bay to San Francisco.
to San Francisco
18 April 1857 · Calif. Stats. 1857, 8th sess., ch. 190/pp. 209-210
1860
Gained territory Marin gained from Sonoma when boundary was changed to run on road; Marin gained Angel Island in San Francisco Bay from San Francisco.
from Sonoma from San Francisco
25 April 1860 · Calif. Stats. 1860, 11th sess., ch. 288/p. 269
1861
Lost territory Marin lost to Sonoma (later repealed).
to Sonoma
13 May 1861 · Calif. Stats. 1861, 12th sess., ch. 346/p. 351
1868
Lost territory Marin lost the Brothers Islands to Contra Costa.
to Contra Costa
26 March 1868 · Calif. Stats. 1867, 17th sess., ch. 305/p. 347

Land and parcels

What the maps cover in Marin

Who publishes each layer, how good it is, and when we last checked.

Parcels Good enough

Lot lines and tax-map numbers for 96,239 parcels from Marin County. Drawn for tax purposes: usually within a few feet of a survey, but not a survey.
edited 3 June 2026 · checked 16 October 2026
Address points Missing

No public address points layer found for Marin County yet.
Buildings Reference

Roof outlines traced from aerial imagery from FEMA Response Geospatial Office with Oak Ridge National Laboratory (ORNL). Good for seeing what is on a lot; not accurate enough to set a fence from.
checked 18 October 2026

Official: from the office that keeps the record
Good enough: a reliable statewide or national copy
Fallback: a guide only; check the official record
Missing: nothing public found yet

People and land

Marin by the Census

Population

262,321 2020 Census count

Housing units

111,564 2020 Census count

Land area

520 square miles

Water area

308 square miles · 37% of the county, including large bodies of water

People per sq mi

504 population ÷ land area

Source: US Census Bureau, 2020 Census; US Census Bureau, 2020 Gazetteer Files. Not endorsed or certified by the Census Bureau.

Rules and offices

Marin County's official site

We have found the county's site but not its offices yet; start there.

County government

County government
The official website of Marin County.
co.marin.ca.us

See your own lot on these maps

Open Marin County on Parcel Chart's free map and click your parcel. Save it with a free account; the full record of your place (site map, projects, documents) is Pro, in early access.

Open the free map no sign-in Pro early access

Questions

Common questions about Marin County

When was Marin County founded?: Marin County was founded in 1850, formed from Sonoma County, and named after Chief Marin.
What is the county seat of Marin County?: The county seat of Marin County is San Rafael.
Is there an online parcel map for Marin County?: Yes. Marin County publishes a parcel map of 96,239 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.
Has Marin County's boundary or name changed?: Marin County was created in 1850 from Sonoma County. Since then the Atlas of Historical County Boundaries records 7 later changes, chiefly involving Sonoma County, San Francisco County and Contra Costa County.
Is my lot in a flood zone?: We do not have flood maps for Marin County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

Nearby