As the American Civil War was in progress, they were able to pose as Confederate Partisan Rangers, and their original mission was to rid the area of (anti-slavery) Republicans.
The two leaders were John Mason, an alleged murderer, and Tom McCauley, a California Gold Rush criminal using the alias Jim Henry.
McCauley was shot dead in September 1865 by the San Bernardino County Sheriff Benjamin Franklin Mathews's posse, and Mason killed in April 1866 by a miner, Ben Mayfield, whom he had tried to kidnap.
In early 1864, a dedicated southern sympathizer from Maryland, secessionist Judge George Gordon Belt, a rancher and former alcalde in Stockton, used his ranch on the Merced River to organize a group of partisan rangers.
When several of the gang were captured and lynched by vigilantes, Tom McCauley then fled and reinvented himself as Jim Henry.
In early 1864 the gang rode over to Santa Clara County, a center of Copperhead sympathizers, to recruit more members.
Unfortunately it was a drought year that depressed the economy, and the increasingly bad war news also discouraged most of their recruits.
On November 10, 1864, the Mason Henry Gang committed their first crimes, three murders, soon after the second election of Abraham Lincoln.
These crimes were described in the Stockton Daily Independent, for Monday, November 14, 1864: On the evening after the election Mason and McHenry went over to Dutch Charley's, against whom they had a spite, and killed him.
[5] Soon after the murders, they held up a stage on the road from Watsonville to Visalia, killing three men and vowing to "slay every Republican they would meet."
Camp commander Major Michael O'Brien, 6th California Infantry, shortly afterward, received intelligence about the location of the Mason Henry Gang hideout.
[7] On February 18, 1865, Captain Herman Noble sent a detachment of Company E, 2nd California Cavalry, under Sergeant Rowley, from Camp Babbitt near Visalia in a long pursuit of men believed to be the Mason Henry Gang.
They report a very hard skirmish, traveling over 900 miles through a most desolate country; upon several occasions going out two or three days without food for themselves, or forage for their horses.
It is believed by many that they have gone to recruit a guerrilla band, and will return to prey on Union men in the lower part of the State.
Doubtless, Visalia would have furnished several birds of prey and a surgeon or two, to bind up their broken bones, and very likely a Chaplain to minister to their bruised souls, and a number of spies, sneaks, and informers.
When word of the attack arrived at San Juan Bautista, Captain Jimeno, of the Native Cavalry, in command of Camp Low, again sent out Lieutenant Lafferty with a detachment of five men to intercept the gang hoping to head them off at Panoche Pass on the western side of the Diablo Range.
In July 1865 Mason and another gang member, Hawkins, pulled guns on Kern River rancher Philo Jewett, who had fed them dinner.
In September of that year, he and his associates were camped out south of San Bernardino and sent John Rogers to town to obtain provisions.
Locals of Union sympathies took note, and Rogers soon found himself in the company of San Bernardino County Sheriff Benjamin Franklin Mathews and his posse, leading them to the outlaw camp.
That night while the three were in the same house, none went to sleep, but in the early morning Mason lay down on his bed under a blanket, but was awake.
The Sacramento Daily Union, June 23, 1866, quoted The Wilmington Journal on the verdict: On the evening of June 8, the jury in the case of Benjamin Ben Mayfield, who murdered the highwayman John Mason, returned a verdict of guilty of murder in the first degree.
The jury again found the defendant guilty of murder in the first degree, and the counsel for the defense asked for a new trial, which was refused by Judge de la Guerra.