William Thomas Clark, Union soldier and legislator, was born in Norwalk, Connecticut, on June 29, 1831, the son of Levi and Fanny Clark. At age thirteen he was forced to quit school and begin to support himself because of his family's extreme poverty. After a few years of doing odd jobs around Norwalk he took a job as a teacher and then began the study of law. Clark moved in 1854 to New York, where he was admitted to the bar. There in 1856 he married Laura Clark of Hartford. The couple immediately moved to Davenport, Iowa, where Clark began his practice. With the advent of the Civil War he raised the Thirteenth Iowa Infantry. On November 2, 1861, he was commissioned a first lieutenant in the regiment and appointed its adjutant. He saw action at the battles of Shiloh, Corinth, Vicksburg, and Atlanta. He was promoted to captain on March 6 and to major on November 24, 1862. On February 10, 1863, he was promoted to lieutenant colonel, and when Gen. James B. McPherson became the commanding officer of the Army of the Tennessee, Clark was named his adjutant general. Clark was brevetted to the rank of brigadier general "for gallant and distinguished service" at the battle of Atlanta on July 22, 1864, and to major general on November 24, 1865, for his distinguished service during the war.
At the end of the war he was transferred to Texas as a part of the United States force sent to check French expansionism in Mexico through the puppet emperor Maximilian. There he became involved in intrigue with Mexican general José Antonio Mexía, who promised to turn over Matamoros to the United States for $200,000. Generals Philip Sheridan and Ulysses S. Grant favored this scheme, but it was vetoed by President Andrew Johnson and Secretary of State William H. Seward.
Clark was discharged from the army on February 1, 1866, and moved to Galveston, where he helped to organize a bank, of which he became cashier. He also became active in the formation of chapters of the Union League and formed a close association with George T. Ruby, a leader of Galveston freedmen. In 1869 he was elected to the United States House of Representatives of the Forty-first Congress from the Third District of Texas. In Congress he labored to sell to the national government a vast region of West Texas for the sum of $40 million to be used to subsidize railroad building and Negro education in the state. Clark was defeated for reelection in 1871 by Dewitt Clinton Giddings despite Governor Edmund J. Davis's flagrant attempt to alter the election results and have him seated. Clark, in fact, returned to Washington and attempted to occupy his old seat but was expelled by the unanimous vote of his colleagues; Giddings was seated instead (see GIDDINGS-CLARK ELECTION CONTEST).
Clark was postmaster at Galveston from 1872 to 1874 and then secured a post in the Bureau of Internal Revenue, at which he served until his death in New York City on October 12, 1905. He is buried in Arlington National Cemetery. Historian Rossetter Gleason Cole referred to Clark as "the last of the carpetbaggers."