Samuel Langhorne Clemens first used this pen name in 1863 while with the Territorial Enterprise in Virginia City, Nev.
In an Enterprise 
article from February 1863, Clemens first used the byline Mark Twain, apparently 
a reference to... [DOC] [TLE] Mark Twain begins reporting in Virginia City - History (history.com) [PAR] Writing under the name of Mark Twain, Samuel Clemens begins publishing news 
stories in the ... a job as reporter for a Virginia City, Nevada, newspaper called 
the Territorial Enterprise.