William received limited education. His school life was cut short at the age of thirteen, when on account of the poverty of his parents he was thrown on his own resources. For a few years he did odd jobs in his home town, then taught school and studied law. In 1854 he removed to New York and was admitted to the bar.
In 1856 Clark went to Davenport, Iowa, where he started to practise law in the firm of Judge Dillon, later one of the counsel for the Gould interests. At the beginning of the Civil War, with the assistance of a chaplain, a drummer, and a fifer, he raised the 13th Iowa Regiment and went to the front, himself as adjutant of the command with the rank of captain. Almost from the beginning he was a staff officer. When General McPherson took command of the Army of the Tennessee he made Clark his adjutant-general, and it was Clark who brought to Sherman the first news of the tragic death of McPherson before Atlanta. As chief of staff, Clark seems to have been energetic and efficient.
In the fall of 1865, now a brigadier-general by brevet (July 22, 1864), he was transferred to Texas, where the Americans were watching the progress of the war against Maximilian. On the Rio Grande he became interested in a scheme by which the liberal general, Mejia, was to hand over Matamoras to the Americans in return for a subsidy of $200, 000. The plan, eagerly advocated by Clark, won the initial favor of Sheridan in New Orleans and of Grant in Washington, both of whom were anxious for intervention against Maximilian, but was blocked by President Johnson and Secretary Seward, who preferred the slower and less dangerous methods of diplomacy.
Clark now resigned from the army, and in 1866 was in Galveston, helping to organize one of the first national banks in Texas, of which he became cashier. About this time he began to take an active part in the creation of union leagues among the African Americans and came into close affiliation with G. T. Ruby, the mulatto leader of the Galveston African Americans.
In 1869, Clark was elected to Congress from the Third District of the now “reconstructed” State of Texas. In Congress his speeches were frank and complete expositions of the political philosophy of the carpet-baggers but had little influence on legislation. His plan to sell a vast region in western Texas to the nation for a sum of forty millions of dollars, to be used to subsidize various railroads (in which Clark was reputed to be interested) and to advance African American education, was regarded by conservative newspapers as a mere attempt to add to the resources of a corrupt administration.
When he came up for reelection in 1871, the conservative forces were for the first time well organized and ably led, and the election resulted in a plurality of more than three thousand votes in favor of the Democratic candidate, D. C. Giddings of Brenham. E. J. Davis, the Republican governor, had, however, small difficulty in changing these figures into a narrow victory for Clark, who was seated pending an investigation. But, largely owing to the wide publicity given the whole matter by Horace Greeley of the Tribune, on May 13, 1872, Clark was expelled by the unanimous vote of a Republican House and Giddings was seated in his stead. The incident was an important step in what was later to be called the “regeneration of the South. ”
At the close of his congressional career, Clark secured a post in the Bureau of Internal Revenue, in which he served until his death in 1905. The old general was justly proud of his army record and his military bearing. For forty years he always wore the same type of slouch hat and the same type of high top boots. In Texas, he will be remembered as “the last of the carpetbaggers. ”
Clark was a member of the Republican Party.
In 1856 Clark was married to Laura Clark of Hartford.