Cal. Gov. Code § 23141
The boundaries of San Mateo County are as follows:
Beginning, at the southwest corner, being the West corner of Santa Cruz; thence on the northwestern line of Santa Cruz to the southwestern line of Santa Clara, being the summit line of the Santa Cruz Mountains; thence northwesterly by said summit line to the source of San Francisquito Creek; thence down the South branch thereof, and down said creek in a general northerly and northeasterly direction along the common boundary between San Mateo County and Santa Clara County to a point where said boundary intersects the D 2 center line of the Bayshore Freeway at California State Highway Engineer's Station D 2 322+68.88 P.O.T. as said Engineer's Station and said common boundary is designated on "AS BUILT"-PLAN AND PROFILE OF STATE HIGHWAY IN SAN MATEO AND SANTA CLARA COUNTIES, BETWEEN 0.3 MILE SOUTH OF WILLOW ROAD AND 0.5 MILE SOUTH OF SAN MATEO-SANTA CLARA COUNTY LINE",-San Mateo, Santa Clara Route 68, Section D, A., accepted June 4th, 1958; said intersection being in the boundary of San Mateo County and Santa Clara County as said boundary was established by Ordinance No. 1619, changing the boundary of said counties, adopted October 15th, 1963, by the Board of Supervisors of the County of San Mateo; thence along said boundary as changed, northeasterly 105 feet to the southwesterly terminus of that certain course "South 40° 47' West" designated "Sta. 78+45± End of Work" as said course and said station are shown on "MAP AND GENERAL CONSTRUCTION PLANS OF ZONE NW-1 "NORTHWEST", PROJECT NO. 2 SAN FRANCISQUITO CREEK IMPROVEMENT FROM BAYSHORE HIGHWAY TO SAN FRANCISCO BAY, SANTA CLARA COUNTY FLOOD CONTROL AND WATER CONSERVATION DISTRICT, SANTA CLARA COUNTY, CALIFORNIA, DATED SEPTEMBER 1958"-said plans on file in San Mateo County Engineer's Office in Case 3, H-1691; thence along the center line of said San Francisquito Creek improvement, North 40° 47' East 128.12 feet, more or less, to Station 77+16.88 of said center line; thence on a curve to the right, said curve having a radius of 305.00 feet and a central angle of 82° 30' an arc distance of 439.17 feet; thence South 56° 43' East 235.00 feet; thence South 61° 43' East 204.00 feet; thence on a curve to the left said curve having a radius of 250.00 feet and a central angle of 49° 00' an arc distance of 213.80 feet; thence North 69° 17' East 404.14 feet; thence on a curve to the left said curve having a radius of 425.00 feet and a central angle of 96° 45' an arc distance of 717.66 feet; thence North 27° 28' West 367.46 feet; thence on a curve to the right said curve having a radius of 455.00 feet and a central angle of 28° 15' an arc distance of 224.38 feet; thence North 0° 47' East 295.00 feet; thence on a curve to the left said curve having a radius of 855.00 feet and a central angle of 13° 37' an arc distance of 203.20 feet; thence North 12° 50' West 1075.00 feet; thence on a curve to the right said curve having a radius of 300.00 feet and a central angle of 65° 35' an arc distance of 343.39 feet; thence North 52° 45' East 1250.00 feet; thence North 50° 20' East 820.00 feet; thence on a curve to the right said curve having a radius of 300.00 feet and a central angle of 20° 00' an arc distance of 104.72 feet; thence North 70° 20' East 500 feet to Station 3+00, designated "Beginning of Project" on said San Francisquito Creek Improvement; thence continuing along last mentioned course North 70° 20' East 300 feet, more or less, to a point in the corporate limits of the City of Menlo Park, as said limits were established by that certain annexation to the City of Menlo Park by Ordinance No. 318, adopted July 14th, 1959, by the City Council of said City; thence southeasterly, easterly and southerly along said corporate limits to the intersection thereof with said common County boundary as the same existed prior to Ordinance No. 1619; thence northeasterly along said boundary to a point in the middle of San Francisco Bay forming a common corner of San Mateo, Santa Clara, and Alameda; thence northerly in a direct line to a point in the center of the ship channel in the Bay of San Francisco West of and opposite to Dumbarton Point; thence northerly in a direct line to the southeast corner of San Francisco; Thence westerly on the boundary line between San Mateo and San Francisco (said line being the North boundary of San Mateo and the South boundary of San Francisco, between San Mateo and San Francisco), as the line was surveyed, established, and marked by Charles S. Tilton, City and County Surveyor of San Francisco, William B. Gilbert, County Surveyor of San Mateo; and D. Bromfield, Assistant Civil Engineer, of San Mateo, between August 28 and December 28, 1898, and being the North boundary line of San Mateo, and the South boundary line of San Francisco, and marked by granite monuments eight inch by eight inch square set three feet in the ground in a bed of concrete three feet square and three in the ground, on section and quarter-section corners, on the township line between T. 2 and 3 S., R. 5 and 6 W., M.D.B. & M., and the line being marked on each monument by a copper nail in a plug of lead which has been countersunk into the top of the monument, and on the dressed faces the letters "S.F." being cut into the stone on the San Francisco side of the line, and the letters "S.M." being cut into the stone in the San Mateo side of the line, and the bearing of the said line being determined by stellar observation as North 89° 491/2' East, to the southwest corner of the said boundary line of San Francisco in the Pacific Ocean, and thence southerly along the ocean shore to the Point of Beginning. The eastern boundary of San Mateo shall be the western boundary of Alameda, insofar as the same borders on San Mateo.
Ca. Gov. Code § 23141