William Thornton Kemper Sr. (November 2, 1867 – January 19, 1938) was an American banker who was the patriarch of the Missouri Kemper family, which developed both Commerce Bancshares and United Missouri Bank to become a major banking family in the Midwest.

Kemper was born on November 2, 1867, in Gallatin, Missouri. He was the son of Sarah Ann Paxton and James Madison Kemper. His mother died in 1875, when he was seven years old.^{[citation needed]} At the age of seventeen, Kemper moved with his family to St. Joseph, Missouri.

Kemper never completed a formal high-school education. At the age of 14 he began work, sweeping the floor at Collins and White, a St. Joseph shoe store, and later became a salesman for Noyes, Norman and Company, a shoe distributor and manufacturer (also in St. Joseph) in which his father was a part owner, covering retailers in eastern Kansas, including Valley Falls, Kansas. In 1885 Kemper moved from St. Joseph to Valley Falls to establish a dry goods store, "Evans & Kemper".

Kemper would spend the next eight years in Valley Falls, where he courted and married Charlotte Crosby and began a career in commercial banking. Valley Falls (then known as "Grasshopper Falls") was a small settlement on the Delaware River about 35 miles due west of Leavenworth, Kansas. In the years following the Civil War, such towns were promoted as commercial hubs in the new network of regional and trans-continental railroads. The northern route of the Atchison, Topeka and Santa Fe railroad was built through Valley Falls in 1871, later joined by the Union Pacific and Missouri Pacific.

Kemper's connection with the Crosby family had both business and personal aspects, and may have predated his move to Valley Falls. When he was traveling through eastern Kansas representing Noyes, Norman, one of Kemper's accounts was with A.D. Kendall, a dry-goods retailer initially established by Charlotte Crosby's father, Rufus Henry Crosby (1834-1891).

Kemper's (future) father in law, Rufus Crosby, was a civic and business leader dating from his arrival in Kansas from Maine in 1855, soon after the opening of Kansas Territory to general settlement under the Kansas-Nebraska Act. An outspoken Abolitionist, Crosby was a signer of the first Kansas State Constitution; he commanded a local free-state volunteer militia; his first store in Valley Falls was burned to the ground in 1856 by Border Ruffians (pro-slavery guerrilla fighters). During his three decades in Kansas, Crosby established and operated a newspaper, a dry-goods store and a bank. In 1879, Crosby started the Valley Falls Bank of Deposit, selling his interest in the store to his partner and brother-in-law, Alvin Kendall.

In 1887, Kemper joined Rufus Crosby's bank as cashier, while also remaining active his other business activities. He married Charlotte Crosby in 1890, and became a partner in the Crosby bank in September 1891. Soon after Rufus Crosby's death in December of that same year, the Valley Falls bank was purchased by A.D. Kendall (and eventually re-chartered as The Kendall State Bank).

Kemper and his wife moved to Kansas City, Missouri, in 1893.