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Just where would that ancestor have lived?

So… an ancestor died in San Mateo County, California, on 1 January 1886. His obituary kindly informs us that he had lived in the same house for exactly 40 years, having moved in on 1 January 1846.

Here’s the question: where exactly would you report that he lived on 1 January 1846? 1856? 1866? 1876?

Most of us would duly enter San Mateo County, California, for each of those dates if we needed to enter them in our databases.

And most of us would be wrong.

Yes, The Legal Genealogist is getting ready to head out to San Mateo for this weekend’s Fall 2019 Seminar of the San Mateo County Genealogical Society. We’ll be talking about “Finding the Law,” “The Law and Reasonably Exhaustive (Re)Search,” “DNA and the Golden Rule: The Law and Ethics of Genetic Genealogy,” and one of my favorites: “No Vitals? No Problem! Building a Family through Circumstantial Evidence.”

And, as always, before heading out, I’m poking around in the laws and records of the area… and finding that things are not always as they seem, geographically.

First off, on the first of January 1846, there was no state of California, and no counties within what became California. At that point, it was still a department of Mexico called Las Californias. It had been a province of New Spain as of 1768, divided into Alta (upper) and Baja (lower) California in 1804, and reunited into Las Californias as a department of Mexico in 1836. When Mexico ceded what became California to the United States under the Treaty of Guadalupe Hidalgo in 1848, it was an unorganized territory.1

So where did the ancestor live on 1 January 1846? In Las Californias, Mexico.

When California’s constitutional convention met in preparation for becoming a state in 1850, 27 counties were originally created: “Effective February 18, 1850, twenty-seven counties were created in California. The new counties were Branciforte, Butte, Calaveras, Colusi, Contra Costa, El Dorado, Los Angeles, Marin, Mariposa, Mendocino, Monterey, Napa, Sacramento, San Diego, San Francisco, San Joaquin, San Luis Obispo, Santa Barbara, Santa Clara, Solano, Sonoma, Sutter, Trinity, Tuolumne, Yola, and Yuba.”2

Before California was admitted as a state, three county names were changed: “Branciforte was changed to Santa Cruz, Colusi was changed to Colusa, and Yola was changed to Yolo.” And on 9 September 1850, California was admitted as a state with those 27 counties. There were no changes in the borders of San Francisco County before the summer of 1856.3

So where did the ancestor live on 1 January 1856? In San Francisco County, California. Or maybe in Santa Cruz County. You see, there was a disputed territory where San Francisco in the north bordered Santa Cruz in the south that wasn’t resolved by 1856.4

San Mateo County itself wasn’t created until 1 July 1856,5 and that southern side border was still unsettled.6 You can see that in the left-most panel of the image below.

It wasn’t until the 18th of April 1857 that the boundaries of San Mateo County were clarified and the border dispute settled — very much in favor of San Mateo County.7 And there were no changes in the county lines again before 1868, so the center panel below shows the boundaries between 1857 and 1868.

San Mateo County

So where did the ancestor live on 1 January 1866? San Mateo County, right? Well… not so fast. Sure he could have lived in San Mateo County. But we still have to consider that 1868 change. Because that change added a very large piece of turf to San Mateo County, also from Santa Cruz County.8 The boundaries set in 1868 are shown in the right-hand panel above.

And it was that definition of the boundaries of San Mateo County that applied in 1876 and 1886.9 Which means that the place in San Mateo where the ancestor lived on 1 January 1886 may not have been in San Mateo County at all before 1868.

Mapping San Mateo through the laws of the day tells us this is a lot harder question than it seemed at first…

Cite/link to this post: Judy G. Russell, “Mapping San Mateo,” The Legal Genealogist ( : posted 30 Oct 2019).


  1. See generally Wikipedia (, “The Californias,” rev. 7 Aug 2019.
  2. Chronology,” County History, California State Association of Counties ( : accessed 30 Oct 2019).
  3. Ibid.
  4. See John H. Long, editor, “California: Individual County Chronologies,” California Atlas of Historical County Boundaries, Newberry Library ( : accessed 30 Oct 2019).
  5. Schedule, §9, in Chapter CXXV, “An Act To Repeal the Several Charters of the City of San Francisco, to Establish the Boundaries of the City and County of San Francisco and to Consolidate the Government Thereof,” in The Statutes of California … 1856 (Sacramento : James Allen, State Printer, 1856), 176; digital images, Office of the Chief Clerk of the California State Assembly ( : accessed 30 Oct 2019).
  6. Long, editor, “California: Individual County Chronologies.”
  7. §2, in Chapter CXCII, “An Act To reorganize and establish the County of San Mateo,” in The Statutes of California … 1857 (Sacramento : James Allen, State Printer, 1857), 222-223; digital images, Office of the Chief Clerk of the California State Assembly ( : accessed 30 Oct 2019).
  8. Ibid., §1, in Chapter CXCV, “An Act to fix and define and boundary line between the Counties of San Mateo and Santa Cruz,” in The Statutes of California … 1867-8 (Sacramento : D.W. Gelwicks, State Printer, 1868), 174-175.
  9. Subsequent boundary adjustments wouldn’t have impacted our research question. See Long, editor, “California: Individual County Chronologies.”
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