Born Samuel Langhorne Clemens in Florida, Missouri, he became more widely known as Mark Twain--the term used on steamboats as a warning that a river's depth is only two fathoms deep, the minimum depth at which a boat could safely navigate. Clemens was a steamboat captain for two years, receiving his pilot's license in 1859 at 23 years old after serving as a pilot's apprentice for a couple of years, until all steamboat traffic was brought to a halt due to the Civil War.
Clemens began developing his style of humorous writing as early as 1857 when the Keokuk Daily Post commissioned him to write a series of humorous travel letters. He served a few months in the Confederate Army as a volunteer, a miserable time for him.
Clemens first used the pen name "Mark Twain" in 1861 while writing a humorous travel letter for the Virginia City Territorial Enterprise and the psuedonym stuck for a half-century.
The story that first made him famous, "The Celebrated Jumping Frog of Calaveras County" was published in 1864 while working as a reporter in San Francisco.
In 1869 he published The Innocents Abroad, a popular book relating his experiences traveling around the world the previous three years.
A year later he married the daughter of a wealthy coal merchant, Olivia Langdon, settling in Hartford, Connecticut. In this setting Twain published The Adventures of Tom Sawyer in 1876, The Prince and the Pauper in 1882, Life on the Mississipi in 1883, A Connecticut Yankee in King Arthur's Court and The Adventures of Huckleberry Finn.
Twain's life then took a turn for the worse after he published Huckleberry Finn in 1885: it took profits from his next three books to recover from bankruptcy--the result of bad investments.
Twain entered a period of dark disillusionment when, in 1903, he and his family moved to Italy, and shortly thereafter she died. His last two books, What is Man? in 1906 and The Mysterious Stranger lacked the carefree lightness of previous works.