Mark Twain (1835–1910) was the pseudonym of Samuel Langhorne Clemens. Clemens took the name from a nautical term for "the second mark," referring to water that was two fathoms deep, or a safe depth for a boat, a term that he encountered as a riverboat pilot. Born in Florida, Missouri, Twain grew up in the nearby river town of Hannibal. At the age of twenty-one, he headed to New Orleans to depart for a trip to the Amazon. The plan fell apart, but Twain found a position as an apprentice riverboat pilot, a prestigious job that fulfilled a childhood dream. Before the Civil War, trade was lucrative on the Mississippi. When the war interrupted that trade and Twain was forced to find another job, he went west and took a job as a reporter for the Sacramento Union.

Twain began to write his most famous books in 1870. The Adventures of Tom Sawyer (1876), Life on the Mississippi (1883), and Adventures of Huckleberry Finn (1883) all draw from his experiences on the Mississippi River. Huckleberry Finn, often referred to as the great American novel, tells the story of a boy named Huck who, together with an escaped slave named Jim, travels down the Mississippi on a raft. Alternately funny and serious, the book is an implicit indictment of racism, showing that beneath outward differences, people have similar feelings and dreams.

In the 1890s, Twain suffered a series of misfortunes, including the death of a daughter, the illness of another daughter and of his wife, and monetary loss due to failed speculative investments. The writings of Twain's last years are bitterly directed at the hypocrisies of his fellow human beings. Twain's severest criticisms, included in The War Prayer and Letters from Earth, were not published until long after his death. Born on the day of the appearance of Halley's comet, Twain died on the day of its reappearance seventy-two years later.