Giddings specialized in civil and probate cases and developed a lucrative legal practice and statewide reputation in state and federal courts before the Civil War. In 1859 he was construction superintendent of the Washington County Railroad. The Giddings brothers arranged a county school-fund loan and contributed financially to make possible completion of the railroad in 1860. In 1862, despite Unionist sentiment, D. C. Giddings enlisted as a private in the Confederate Twenty-first Texas Cavalry (First Texas Lancers). He was elected captain and then lieutenant colonel. In absence of Col. William Carter, Giddings was actual commander of this regiment during the war. He was briefly captured and exchanged in 1862. He participated in Arkansas and Louisiana campaigns and John S. Marmaduke's Missouri raid.
In 1867 Giddings aided yellow fever victims in Brenham; the same year, he was elected foreman of Hook and Ladder Company No. 1, which, despite its name, was organized to resist the actions of Union troops. Giddings was a member of the Constitutional Convention of 1866. He served on the Resolutions Committee of the conservative state convention in 1868. As a Democrat, he won the 1871 special election for United States congressman from the Third Texas District, in part because of his efforts to gain broad ethnic support. After Republican governor Edmund J. Davis certified the reelection of his opponent, William T. Clark, Giddings won his appeal to the United States House of Representatives, which unanimously seated him in 1872. He was the first Southern Democrat to enter Congress during Reconstruction. He was reelected to the Forty-third Congress and as an advocate of silver defeated independent candidate George Washington Jones to serve in the Forty-fifth Congress (1877-79).
After the Civil War Giddings and his brother J. D. became land agents and owners of holdings throughout Texas. They founded the Giddings and Giddings bank at Brenham in 1866. Dewitt Giddings earned a large commission during Governor Richard Coke's term when he successively recovered $339,000 in proceeds from state-owned bonds sold in Europe during the war. After his brother's death, Giddings managed bank operations and in 1884 became sole owner of the Giddings bank. By 1874 he was a large stockholder in Texas Mutual Life Insurance of Galveston. He chartered the short-lived Brazos Valley, Brenham and Gulf Railway Company in 1888 to promote lower railroad rates. His activities focused on banking after 1875.
Giddings was a Texas presidential elector at large in 1876, a member of the Platforms and Resolutions Committee at Texas Democratic conventions in 1884, 1888, 1892, and 1894, and a Texas delegate to the national Democratic convention in 1884, 1888, and 1892. In 1886 he ran unsuccessfully against Lawrence Sullivan (Sul) Ross for the Democratic gubernatorial nomination. Giddings campaigned against a proposed state prohibition amendment and was chairman of the Anti-Prohibition State Convention in May 1887. As an opponent of Governor James S. Hogg's reelection, Giddings was chairman of the June 1892 state Democratic platform committee, coauthor of the committee's minority report opposing free silver at the Car-Stable Convention (August 1892), and member of the Turner Hall Convention platform committee. In August 1894 he supported the national Democratic party platform as chairman of the state Democratic platform committee. He was a delegate of the Texas Gold Democratic Conference to the Memphis Convention (1895) and delegate at large of Texas Gold Democrats to the Indianapolis Convention. He also served on the state Deep Water Convention Resolutions Committee to promote federal appropriations for a Gulf of Mexico port in 1888. In the 1880s he supported Populists within Washington County to destroy Republican domination of county politics. Giddings was the leading proponent of the establishment in Brenham of the state's first public schools.
In 1860 he married Malinda C. Lusk, the daughter of Samuel C. Lusk. They had five children. Mrs. Giddings died in 1869. Giddings died of heart disease in Brenham on August 19, 1903, and was buried in Prairie Lea cemetery.
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Prairie Lea Cemetery