Mark Twain first used the pen name “Mark Twain” in 1863. Samuel Clemens adopted it while working as a journalist for the Territorial Enterprise in Virginia City, Nevada.
The first known use came on February 3, 1863, when Clemens signed a humorous letter about a lavish party with “Mark Twain.” The name soon became permanently associated with him, although he had previously written under other names, including “Josh” and “Thomas Jefferson Snodgrass.”
“Mark twain” was a riverboat expression meaning that the measured depth was two fathoms, or 12 feet. That depth was generally considered safe for a steamboat. Clemens had worked as a Mississippi River pilot, so the phrase was closely connected to his earlier life. He also said that the name had been used by Captain Isaiah Sellers, a writer of river news, before Clemens adopted it.
The year is sometimes confused with 1865, when Twain's story “The Celebrated Jumping Frog of Calaveras County” brought him wider attention. His national literary fame came later, but the pen name itself first appeared in 1863.