Lake County was founded in 1861, at the insistence of the growing number of settlers in Lower Lake, Lakeport, Upper Lake and Kelseyville.

Its western boundary is delineated by the peaks of the Mayacamas mountain range. Mt. St. Helena marks the extreme of its southern border. Other mountains of the Coast Range ring the northern and eastern county lines, so the county has been considered rather inaccessible.

Despite at least 23 attempts in the late 19th and early 20th century, the railroad never came to Lake County, nor did the freeways. For an area only 100 miles from San Francisco, Lake County remains surprisingly rural. The 1870 U.S. Census  tallied 2,969 residents. In 2010, that had grown only to 64,665.  

In 1858, residents persuaded Napa County to form the Clear Lake Township in the portion of that county north of Calistoga. 

This became the nucleus of Lake County, with bits and pieces of adjoining counties added over the years. At the time of Lake County's birth, there were but a handful of settlers south of Lower Lake.

The Asbill and Copsey families were first to settle in Jerusalem Valley; John Cobb and several other families had settled on 

Cobb Mountain, and a few scattered "squatters" had forbidden homes on the vast expanses of the two ranchos.

The 30,000+ acres comprising the two land grants, Rancho Callayomi and Rancho Guenoc, contained almost all of the tillable valley land south of Lower Lake.

Those two properties were held in probate for 14 years after the death of  owner A.A. Ritchie, in 1856. Callayomi was cleared for sale late in 1870 and Rancho Guenoc two years later.