How did Mark Twain get his name?

Samuel Clemens changed his name to Mark Twain because of his experiences of being on a riverboat.

"Mark twain" was what the leadsman on a riverboat called when the water was two fathoms deep -- that's 12 feet, which is deep enough to be considered safe for most boats of the era.

In addition to a phrase commonly heard on the Mississippi, "Mark Twain" was the original pen name of Captain Sellers, an old steamboat pilot who wrote rather all-knowingly about river conditions for the New Orleans Picayune in the mid-1800s. In one of his early newspaper articles, Samuel Clemens parodied the first Mark Twain's writing. Captain Sellers read the parody and was offended, which Clemens regretted. As a tribute to the old man and steamboat traditions, Clemens started using the pen name of Mark Twain in 1863.

Mark Twain became both Clemens' writing name and a persona he affected in his works. Particularly in his travel books "Innocents Abroad" and "Roughing It," "Mark Twain" narrates as an excitable, naive fellow, quite unlike the real Clemens. In later years, the public figure of Mark Twain became known as a genial humorist, disguising Clemens' serious side.