From 1836 to 1838 Horton, a Democrat, served as senator in the First and Second congresses of the republic, representing Matagorda, Jackson, and Victoria counties. He campaigned unsuccessfully for the vice presidency in 1838. In January 1839 he was chosen by the Republic of Texas Congress to chair the committee to select the site of the new capital. On March 7, 1842, Horton was recruited to serve as captain under Colonel Owen, to defend against Rafael Vásquez, and his force of 500–700 Mexican soldiers, who had seized San Antonio. Horton served as a delegate to the Convention of 1845 and subsequently consented to run for lieutenant governor. Voting returns initially awarded the victory to his opponent, Nicholas Henry Darnell, but late returns from several South Texas counties were sufficient to alter the results. On May 1, 1846, Horton was declared the first lieutenant governor of the new state.
When Governor James Pinckney Henderson left to assume command of Texas volunteers assembled to deal with troubles with Mexico, Horton served as governor pro tem from May 19, 1846 until Henderson returned on November 13, 1846. He was never elected to another public office, and he emerged from retirement only to attend the Democratic national convention in Charleston in 1860 and the state Secession Congress in 1861. Horton moved to his plantation on Caney Creek in what is now Wharton County, near Wharton, by 1843 and maintained a large home on a plantation in Matagorda County, near the town of Matagorda. On the eve of the Civil War he owned more than 150 slaves and was one of the wealthiest men in the state. During the war, however, he lost most of his money. He was a lifelong Baptist and an original member of the board of trustees that established Baylor University. Of the six children born during his marriage, only a son and a daughter survived him. Horton died on September 1, 1865, at Matagorda, where he is buried.
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