Chamberlain was born in Center Harbor, New Hampshire, to Ephraim Chamberlain and soon afterward moved to Boston, Massachusetts, where he spent most of his childhood. In 1844, at age 15, he left home without permission to go toIllinois. Two years later he joined the 2nd Illinois Volunteer Regiment and headed to Texas to participate in the Mexican–American War. In San Antonio, Chamberlain joined the regular army and became part of the 1st U.S. Dragoons. He fought at the Battle of Buena Vista in February 1847 as well as several other operations in Mexico. In 1849, he was declared a deserter and by 1854 he had returned home to Boston to raise a family. He married Mary Keith on July 4, 1855, and they had three children. By his own account, Chamberlain was also involved in some less savory aspects of the Texas–Mexico border disputes. Most notably, he rode for a time with the infamous Glanton gang under the command of John Glanton, which was contracted by Mexican authorities to track and kill hostile Apaches following the war. The gang became notorious for taking scalps under highly questionable circumstances to claim the bounties, and its members were eventually declared outlaws by the Mexican government. Chamberlain's memoirs regarding this period are the only known primary source attesting to the existence of the Texian known as Judge Holden, Glanton's second-in-command. While Chamberlain and some others escaped, Glanton and a number of his gang were massacred by Yuma Indians near what is now Yuma, Arizona, in April 1850.
After the war, Chamberlain was a warden of state prisons in Massachusetts and Connecticut. Settling in Massachusetts with his family, Chamberlain became well-known for his charming paintings, which consist largely of landscapes and battle scenes depicting the Mexican–American War. A large collection is held at the San Jacinto Museum of History. Chamberlain also authored and illustrated a harrowing autobiographical account of his travels during the 1840s, especially his service in the Mexican–American War and his adventures with the Glanton gang, entitled My Confession: The Recollections of a Rogue, which he wrote between 1855 and 1861. Though many of its stories are exaggerated in the romantic literary style popular at the time, research has corroborated most of them as true. Noted for its authentic descriptions of the experiences of the typical American soldier during the Mexican–American War, Chamberlain's account continues to serve as a valuable primary source text for historians studying the war and the era as a whole. The account also served as the basis for author Cormac McCarthy's 1985 novel Blood Meridian; the novel's protagonist, known only as "the kid", is said to be loosely based on Chamberlain. Chamberlain died on November 10, 1908 in Worcester, Massachusetts, and was buried in Mount Auburn Cemetery.
