In 1856 Roberts ran for and won a position on the Texas Supreme Court, where he joined his friend Royal T. Wheeler, the chief justice. During this time Roberts became a spokesman for states' rights, and when the secessionist crisis appeared in 1860, he was at the center of the pro-Confederate faction. In January 1861 he was unanimously elected president of the Secession Convention in Austin, a meeting that he had been influential in calling. Along with East Texas colleagues George W. Chilton and John S. Ford, Roberts led the passage of the ordinance removing Texas from the Union in 1861. In 1862 he returned to East Texas, where he helped raise a regiment, the Eleventh Texas Infantry of Walker's Texas Division. His military career was brief. After seeing very little combat and after an unsuccessful attempt to gain a brigadiership, Roberts returned to Austin as chief justice of the Texas Supreme Court in 1864. He held this position until he was removed along with other state incumbents in 1865.
During Reconstruction he was elected a delegate to the Constitutional Convention of 1866 and also, along with David G. Burnet, was elected United States senator. As Roberts had anticipated, the new majority of Radical Republicans in Congress refused to seat the entire Texas delegation along with the delegations of other southern states. After his rejection, about which he later wrote an article entitled "The Experience of an Unrecognized Senator," published in the Quarterly of the Texas State Historical Association (now the Southwestern Historical Quarterly) in 1908. Roberts eventually returned to Gilmer, Texas, where he opened a law school in 1868. Among his students were a future Texas Supreme Court justice, Sawnie Robertson, and a Dallas district judge, George N. Aldredge. With the return of the Democrats to power in Austin in 1874, Roberts was first appointed, then elected, to the Texas Supreme Court. He served as chief justice for four years and was involved in rewriting much of Texas civil law. In 1878 he was elected governor of Texas on a platform of post-Reconstruction fiscal reform. His two gubernatorial terms were marked by a reduction in state expenditures. His plan for countering the high taxes and state debt of the Reconstruction years became known as "pay as you go." A major part of this plan involved the sale of public lands to finance the debt and to fund public schools. Though ultimately successful in both reducing the debt and increasing the public school fund, the decreased government appropriations under Roberts halted public school growth for a time. Also, his land policy tended to favor large ranchers and companies in the development of West Texas. Nonetheless he remained popular with rural landowners, largely because he lowered taxes, as well as with land speculators. The present Capitol in Austin was contracted during Roberts's terms, and the cornerstone for the University of Texas was laid in 1882. Railroad mileage increased across West Texas, and the frontier became more secure.
In 1883, shortly before Roberts's term as governor ended, the University of Texas opened in Austin. Upon his retirement Roberts was immediately appointed professor of law, a position he held for the next ten years. During this period he was immensely influential in the state's legal profession. His impact on a generation of young attorneys was symbolized by the affectionate title "Old Alcalde" bestowed on him by his students. During his tenure at the university, Roberts wrote several professional works, among them a text, The Elements of Texas Pleading (1890), which was used for decades after his retirement from teaching. In 1893 he left the university and moved to Marble Falls, where he turned his attention to more general historical writings.
His essay The Political, Legislative, and Judicial History of Texas for its Fifty Years of Statehood, 1845-1895 was published in an early general history of the state, Comprehensive History of Texas, 1685 to 1897 (1898), edited by Dudley G. Wooten. Roberts's chapters on Texas in volume eleven of C. A. Evans's Confederate Military History (1899) stress the role of the Lone Star State in the Civil War. With his interest in Texas history unabated, Roberts returned to Austin in 1895. Here, along with several other prominent Texans, he participated in forming the Texas State Historical Association. He served as the organization's first president and submitted several of the first articles published in its Quarterly. Roberts was married to Francis W. Edwards of Ashville, Alabama, from 1837 until her death in 1883. They were the parents of seven children. In 1887 Roberts married Mrs. Catherine E. Border. He died at his home in Austin on May 19, 1898, and was buried in Oakwood Cemetery.
30° 16.534, -097° 43.483