Samuel Clemens first adopted the pen name 'Mark Twain' in 1863.
Notes on verification
Corroborated by multiple reputable, independent sources including the Mark Twain House & Museum, Encyclopedia Britannica, and PBS, consistent with established Twain scholarship.
Sources
- Unswerving irregularity | USPTO (uspto.gov)
- https://www.marktwainhouse.org/about/mark-twain/mark-twain-biography/ (marktwainhouse.org)
- https://www.pbs.org/marktwain/l_mt/l_mt_pseudonym.html (pbs.org)