The Person
The Handbook of Texas Online has an excellent biography on the city namesake:
(1825–1881). Robert H. Cumby, Texas legislator and Confederate Army officer, was born on August 24, 1825, in Charlotte County, Virginia, the son of Morgan and Levicy (Tanner) Cumby. In 1836 he moved with his family to Lafayette County, Mississippi. There he married Nancy L. Zollicoffer and resided until 1849, when he moved to Rusk County, Texas. He became a prominent planter and one of the wealthiest men in the county; in 1860 he owned $22,600 in land and $38,000 in personal property, including thirty slaves. He was elected to the Eighth Texas Legislature in 1859 and served as a representative until 1861. At the outbreak of the Civil War he raised Company B of Col. Elkanah Greer's Third Texas Cavalry, also known as the South Kansas-Texas Regiment. The regiment was organized at Dallas on July 13, 1861, and after serving under Gen. Benjamin McCulloch in Arkansas and Missouri was transferred east of the Mississippi River and became part of Ross's Texas Brigade. When the regiment's one-year enlistment period expired on May 8, 1862, new elections for officers were held, and Cumby was elected colonel. Other outstanding officers in this regiment included M. D. Ector, Walter P. Lane, Hinche P. Mabry, and George W. Chilton. Because he was in poor health with chronic diarrhea, Cumby could not assume command and deferred to Lane. He resigned from command of the regiment on June 12, 1862, and returned to Texas. In 1863 Cumby was appointed major and commissary officer to General Greer. Several politicians, including the Texas congressional delegation, recommended Cumby for promotion to brigadier general, but he never again took the field. In 1864 he was appointed brigadier general, Fourth Brigade, Texas State Troops.
After the war he moved to Dallas, where he served as a constable. Later he moved to Sulphur Springs where he ran a grocery. He died in Sulphur Springs November 19, 1881, and is buried in that town’s city cemetery. Cumby, Texas, was named for him.
The Place
A Texas Historical Marker provides some background:
The grove of black jack trees which gave this town its original name was near an Indian camping ground. The trees stood atop the highest point in present Hopkins County. An Indian trail that crossed the grove later became a major wagon freighting route.
Black Jack Grove Post Office opened in 1848 in the home of John W. Matthews, the first postmaster. In 1851 D.W. Cole purchased a tract of land that included the grove from Elizabeth Wren and began selling town lots. Cole operated a store and gave land for the first Masonic lodge building. Henry Bingham ran a tavern which also served as a hotel. By 1860, Black Jack Grove was a thriving settlement with several stores, two physicians, two blacksmiths, and other tradesmen. A log structure housed the school. Later a vacant store served as a school and church facility.
To avoid confusion with another Black Jack Post Office, the town was renamed in 1896 for legislator and Confederate veteran Robert H. Cumby. After 1900, two banks opened and the town incorporated. Later the population began to decline as improved highways lessened Cumby's importance as a trading center.
One of Cumby's best-known citizens was the noted author Ben K. Green (1912-1974)