What was Mark Twain's Nickname?

Answer 1

Mark Twain is actually the pen name or nickname for his real name Samuel Langhorne Clemens. So in effect Mark Twain is his nickname.

Answer 2

Mark Twain's nickname was actually his pen name; his real name was Samuel Clemens. He chose the name "Mark Twain" from a riverboat term meaning the water is two fathoms deep, indicating it was safe to navigate.