Marcus Alonzo Hanna (September 24, 1837 – February 15, 1904) was an American businessman and Republican politician who served as a United States Senator from Ohio as well as chairman of the Republican National Committee. A friend and political ally of President William McKinley, Hanna used his wealth and business skills to successfully manage McKinley's presidential campaigns in 1896 and in 1900.

Hanna was born in New Lisbon (today Lisbon), Ohio, in 1837. His family moved to the growing city of Cleveland in his teenage years, where he attended high school with John D. Rockefeller, who became a lifelong friend. He was expelled from college, and entered the family mercantile business. He served briefly during the American Civil War and married Charlotte Rhodes. Her father, Daniel Rhodes, took Hanna into his business after the war. Hanna was soon a partner in the firm, which grew to have interests in many areas, especially coal and iron. He was a millionaire by his 40th birthday, and turned his attention to politics.

Despite Hanna's efforts on his behalf, Ohio Senator John Sherman failed to gain the Republican nomination for president in 1884 and 1888. With Sherman becoming too old to be considered a contender, Hanna worked to elect William McKinley. In 1895, Hanna left his business career to devote himself full-time to McKinley's campaign for president. Hanna paid all expenses to get McKinley the nomination the following year, although he was in any event the frontrunner. The Democrats nominated former Nebraska Congressman William Jennings Bryan, who ran on a bimetallism, or "Free Silver", platform. Hanna's fundraising broke records, and once initial public enthusiasm for Bryan and his program subsided, McKinley was elected comfortably.

Declining a Cabinet position, Hanna secured appointment as senator from Ohio after Sherman was made Secretary of State; he was re-elected by the Ohio General Assembly in 1898 and 1904. After McKinley's assassination in 1901, Hanna worked for the building of a canal in Panama, rather than elsewhere in Central America, as had previously been proposed. He died in 1904, and is remembered for his role in McKinley's election, thanks to savage cartoons by such illustrators as Homer Davenport, who lampooned him as McKinley's political master.

Marcus Alonzo Hanna was born on September 24, 1837, in New Lisbon (in 1895 renamed Lisbon), Ohio, to Dr. Leonard and Samantha Hanna. Leonard's father, Benjamin Hanna, a Quaker of Scotch-Irish descent, was a wealthy store owner in New Lisbon. Dr. Hanna practiced in Columbiana County, where New Lisbon was located, until he suffered a spinal injury while riding. After the accident, he joined the family business, B., L., and T. Hanna, by then a major grocery and goods brokering firm. Samantha, née Converse, and her parents had journeyed west from Vermont when she was 11; she was of English, possibly Irish, and French Huguenot descent.

Mark's uncle Kersey Hanna described Mark as a boy as "short, strong and rugged, with a full round figure". Young Mark attended the local public school, which conducted class in the basement of the Presbyterian church. He competed in the local boys' debating society, and on the question of whether the black man had more cause for complaint than the Indian, carried the day arguing for the blacks.

Members of the Hanna family invested in a canal project to connect New Lisbon, distant from waterways, to the Ohio River. The canal was a failure, and the family lost large sums of money. Most Hanna family members left New Lisbon in the early 1850s. Dr. Hanna went into partnership with his brother Robert, starting a grocery business in Cleveland, and relocated his family there in 1852. In Cleveland, Mark attended several public schools, including Cleveland Central High School, which he went to at the same time as John D. Rockefeller and was one of his classmates. After graduation in 1857, Hanna attended Western Reserve College, but was dismissed for distributing mock programs at a solemn ceremonial. Hanna served in various capacities in the family business, learning it from the bottom up.

By the start of the Civil War, he was a major participant in the business. Dr. Hanna had fallen ill with complications from his spinal injury (he died on December 15, 1862), and Mark Hanna, even before his father's death, was made a partner.