Martin D. Hart, state senator and Unionist, the son of Capt. John Hart, was probably born in Indiana in 1821. The family moved to Texas around 1833 and settled first at Jonesboro and later in Warren. When John Hart organized the company known as Hart's Mounted Men in 1836, his fifteen-year-old son, Martin, joined. They served in the Army of the Republic of Texas for three months, from July 20 to October 20, 1836. On March 10, 1842, Martin Hart married Mary Ann Green in Fannin County. The couple eventually had five children. In 1849 Martin and his brother Hardin moved to Hunt County, where they operated a successful law office. By 1860, according to the Hunt County tax rolls, Hart, with more than 5,000 acres valued for tax purposes at $20,542, was the second richest man in the county.
He was elected to the Texas House of Representatives, where he served from 1857 to 1859. He was then elected state senator. During the election of 1860 he supported John Bell, presidential candidate of the Constitutional Union party. After the election, as the thirst for secession swept through Texas, Hart publicly opposed the movement in a series of speeches he made in the county courthouse. When Governor Sam Houston called the Texas legislature into special session in January 1861, Hart traveled to Austin. There he signed the Unionist "Address to the People of the State," which attempted to counter the secessionist argument. After Texas secession in February 1861, he resigned his Senate seat and returned to Hunt County. He resumed his law practice and, in July of 1861, organized and was elected captain of the Greenville Guards, a company of mounted volunteers. He wrote Governor Edward Clark, pledging the company's services "in defense of Texas whenever she is invaded or threatened with invasion." In the summer of 1862 he received a Confederate commission with permission to raise a company and operate in northwest Arkansas. Using his commission to travel through Confederate lines, he and his followers marched to southwest Missouri, where they apparently received Union army papers. Hart returned to Arkansas, led a series of rear-guard actions against Confederate forces, and is alleged to have murdered at least two prominent secessionists. He and some of his followers were captured by Confederate troops on January 18, 1863, and taken to Fort Smith, where he and his first lieutenant, J. W. Hays of Illinois, were court-martialed and hanged, on January 23, 1863. They were buried in unmarked graves under the tree where they were hanged. In 1864, when the federals took Fort Smith, Hart's body was exhumed and reinterred in the national cemetery there. Contributions from Unionists and federal soldiers purchased a headstone.