California became a territory of the United States of America on February 2, 1848 by the treaty of Guadalupe Hildago. It was the news of the discovery of gold later that same year, however, that resulted in the explosion of California's population. Prior to the discovery of gold, the population was estimated at 15,000 persons. The news of the gold discovery quickly circled the globe and hopeful miners of every nationality migrated to the California gold fields in hopes of finding their share of the riches in the precious metal. By the end of 1849 the population had reached 100,000. In 1852 it was 224,000, in 1860 the official census showed 380,000, in 1870 the population had reached 650,000, and by 1880 it was 865,000.
The administration of government with this quick growth required that formal voting procedures be implemented to create voter districts and avoid fraud. Voter registration became necessary and the "Great Registers of Voters" were created. The earliest Great Registers were produced in 1866 as a result of the enactment of Chapter CCLXV, Laws of the 16th Session of the California State Legislature; 1865-1866. This legislation, approved 19 March 1866, and known as the Registration Act, provided "for the registration of the citizens of the State, and for the enrollment in the several election districts of all the legal voters thereof, and for the prevention and punishment of frauds affecting the elective franchise."
Section 3 of the Act required that "The manner of registration shall be as follows: Clear and distinct entries shall be made in said books, setting forth in separate columns the name at full length (without the use of initials) of the person registered; his age, omitting fractions of years; the country of his nativity; his occupation; the particular city, town, township, ward, or district of his residence; if a naturalized foreigner, when, where, and by what Court he was admitted to become a citizen of the United States; also the date of registry, and a number affixed to each name, which numbers shall run successively in the other of registration; and to the truth of the facts stated in such entries the person registered shall be duly sworn, which shall also be noted and verified by the word "sworn", to be entered in a separate column opposite his name.
Each county government was directed to establish a Great Register for the population of their county. The various county Great Register of Voters resulting from this legislation contain a wealth of information concerning the citizens, especially the naturalized citizens of California.
The names of foreign-born voters contained in this statewide compilation were derived from the printed county level Great Registers required by this Act. The year 1872 was selected because that was the earliest year of registration in which most of the State of California's existing counties had complied with the Act and have surviving lists. In some cases the 1873 and 1875 Great Register was used. This was done when the county did not have a 1872 register or when the 1872 register did not contain naturalization data but the next register did.
In 1872 the State of California was comprised of fifty counties. Of these fifty, only one county, Klamath no longer exists, its territory now part of Del Norte and Humboldt counties. Since 1872, nine new counties have been formed.