Thomas Mason Dennis, soldier and legislator, was born in Georgia on March 9, 1807, and moved to Texas in March 1835. He settled in Bastrop and in September joined Capt. Robert M. Coleman's company of volunteers that marched from Bastrop to the relief of Gonzales (see GONZALES, BATTLE OF). On December 20, 1835, he signed the Goliad Declaration of Independence. He enlisted in the Texas army on February 28, 1836, and at San Jacinto served in Capt. Jesse Billingsley's Company C-also known as the "Mina Volunteers"-of Col. Edward Burleson's First Regiment, Texas Volunteers. According to his obituary he was promoted to lieutenant, but no documentary evidence of this promotion has been located. He left the service on June 1.
After his discharge Dennis moved to Matagorda County, where he was elected clerk of the county court in February 1837. He was reelected on February 7, 1839. In 1841 he was elected to represent Matagorda County in the House of Representatives of the Sixth Congress of the republic. There he served on the Naval Affairs Committee. Dennis is said to have been elected captain of several volunteer companies that saw action against the Indians on the western frontier and to have served at the battle of Plum Creek, in the repulse of Rafael Vásquez and Adrián Woll, and on the Somervell expedition. He served as a private in Capt. Albert C. Horton's company in Col. Clark L. Owen's regiment of volunteers from March 6 through April 13, 1842. On February 3, 1845, he was elected sheriff of Matagorda County. By 1850, in partnership with surveyor Joshua Threadgill, he was well known as a horse and general stock raiser; his property was valued at $5,000.
After leaving Matagorda County about 1852 he lived for a time in Gonzales County, where, on September 24, 1852, he was appointed to the county Democratic committee. He is said to have resided in Rockport for some twenty years before moving to Karnes and Wilson counties, where he was engaged in stock raising. By 1871 he was drawing a pension as a veteran of the Texas Revolution. In 1876 he was elected to the House of Representatives of the Fifteenth Legislature from the Seventy-eighth District. He resigned during the session but succeeded himself when he was reelected in the special election. He died the following year on October 15 at the Gonzales County community of Rancho. His will was signed on October 12, 1877, and opened for probate on November 6, 1877. Dennis never married. He was a Democrat and a member of the Texas Veterans Association.