He attended public school until the start of the American Civil War, then took his first cowpuncher lessons in 1867 before moving to St. Louis after his mother remarried.

Siringo attended Fisk public school for a time while in New Orleans but then started work as a cowboy for Abel Head "Shanghai" Pierce in April 1871, after returning to Texas.

Siringo stopped working for the LX Ranch when he married Mamie in 1884 and opened a tobacco store in Caldwell, Kansas.

He began writing his autobiography, A Texas Cow Boy; Or Fifteen Years on the Hurricane Deck of a Spanish Pony.

In February 1891, assuming the name Charles T. Leon, Siringo undertook a 6-month investigation for New Mexico Governor L. Bradford Prince.

Siringo was able to infiltrate Las Gorras Blancas and the Knights of Labor, while understanding their relationship with the Santa Fe Ring.

Located north of Arroyo Chamiso, Siringo built a two-room adobe home, with a view of the Sangre de Cristo Mountains.

[8] After 14 months, which included the Coeur d'Alene, Idaho labor strike of 1892, Siringo's undercover work and testimony helped convict 18 union leaders.

[1]: 48, 55 For 4 years starting in 1899, posing under the aliases "Charles L. Carter", an alleged Mexican outlaw on the run from the law for murder, "Chas.

Siringo called Butch Cassidy, "the shrewdest and most daring out law of the present age," and the Wild Bunch "kept a system of blind post offices all the way from the Hole-in-the-Wall in northern Wyoming to Alma in southern New Mexico, these post offices being in rocky crevices or on top of round mounds on the desert."

In Siringo's words, "I closed the Union Pacific train robbery case after having traveled more than 25,000 miles by rail, vehicles, afoot, and on horseback, and after being on the operation constantly for about four years.

[7][1]: 59–60 [2]: 92–93, 120–148 On that case, Siringo often coordinated with Tom Horn, who was by that time working for large cattle companies as a stock detective, but who also was retained by the Pinkerton Agency on contract to assist in the robbery investigation.

The Pinkerton Detective Agency delayed publication for two years, feeling it violated the confidentiality agreement that Siringo had signed upon hiring.

Siringo wrote that he had been instructed to commit voter fraud in the re-election campaign of Colorado Governor James Peabody.

"[12] The Pinkerton Agency succeeded in suppressing the book, charging Siringo with criminal libel, and calling for his arrest and extradition to Chicago.

[1]: 84–85 In 1916, Siringo began working as a New Mexico Mounted Patrolman to assist in the capture of numerous rustlers in the area, holding that position until 1918.