In the summer of 1838 he immigrated to Texas and established a legal practice at Washington-on-the-Brazos. In early 1840 the Congress of the Republic of Texas elected him judge of the Fourth Judicial District, an election that automatically made him an associate justice of the republic Supreme Court. He was confirmed in the office on January 20, 1840. On March 19, 1840, he participated in the Council House Fight in San Antonio. In 1840-41 Hemphill joined several campaigns against the Comanches, and in 1842-43, during a period when the Supreme Court did not meet, he served as adjutant general of the Somervell expedition. On December 5, 1840, the Congress elected him chief justice of the Supreme Court, a position he held until 1858. He was elected a delegate from Washington County to the Convention of 1845, where he cast his vote in favor of statehood. Governor James Pinckney Henderson appointed Hemphill to a six-year term as chief justice, and he was confirmed on March 2, 1846. After the selection of Supreme Court justices was transferred to the voters, Hemphill was elected chief justice on August 4, 1851, and again in 1856.
As a jurist he took a particular interest in cases involving Spanish and Mexican law, which he had studied intensively, as well as those concerning marital rights, divorce, and homestead and other exemptions. He was noted for the "liberal construction" he placed on married women's rights and for his championship of homestead rights. His decisions are credited with substantially shaping the "form and content" of community property and homestead exemption law. Hemphill regretted the adoption of common law by the Texas Congress in 1840 and managed, in his written opinions, to preserve "something of the liberal spirit of the civil law." He was called the John Marshall of Texas for the significant role he played in the development of Texas jurisprudence.
In November 1857 Democrats, dissatisfied with Sam Houston, nominated Hemphill to succeed Houston when the latter's senatorial term ended in March 1859. Hemphill was subsequently elected by the Texas Senate and took office on March 4, 1859. In January 1861 he delivered an address expressing his belief in the right of states to secede, and on January 6, 1861, he was one of fourteen senators who recommended the immediate withdrawal of the southern states. On February 4, 1861, the Secession Convention elected him one of seven Texas delegates to the convention of Southern states in Montgomery, Alabama, which became the Provisional Confederate Congress. He was subsequently expelled from the United States Senate by resolution on July 11, 1861.
As a member of the Provisional Confederate Congress, Hemphill served on the Commercial and Financial Independence, Finance, and Judiciary committees and on the special committee to digest the laws. He devoted much of his attention to the task of adapting United States laws to Confederate purposes. In November 1861 he ran for a seat in the First Regular Congress but was narrowly defeated by Williamson S. Oldham. Before the end of the Provisional Congress Judge Hemphill died in Richmond, on January 4, 1862. His body was returned to Austin for burial in the State Cemetery. Never married, Hemphill was characterized as a private and reserved yet generous individual. Hemphill County, established on August 21, 1876, was named in his honor.
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Texas State Cemetery