Samuel Langhorne Clemens first used this pen name in 1863 while with the Territorial Enterprise in Virginia City, Nev.
In Nevada, Sam Clemens became a 
miner, ... in Virginia City for the Territorial Enterprise, where he adapted the pen 
name... [DOC] [TLE] Origins of the name Mark Twain | Twain, Mark | Britannica.com [PAR] Although Samuel Clemens's earliest use of the pseudonym Mark Twain has been 
... first used it in February 1863 in the Virginia City Territorial Enterprise.