Politicians at the start of the Civil War began raising volunteer troops in response to President Abraham Lincoln's call to arms to force the return of the Confederate States of America to the Union.

In the late summer of 1862, a small number of men in California who had been raised in the East decided to enlist in the army, but wanted to serve in the Eastern Theater.

They contacted Governor John Andrew of Massachusetts in the late summer of 1862 and offered to raise a company of 100 cavalrymen to serve in a state regiment.

The so-called "California Hundred" was organized in San Francisco on December 10, 1862, and took a ship to the East Coast, arriving at Camp Meigs in Readville, Massachusetts, on January 4, 1863.

The Californians and four of the local companies were moved in mid-February to Baltimore, Maryland, then to Fort Monroe and numerous other locations in Virginia, where they primarily engaged in picketing, scouting, and outpost duty until July 1863.