Robert Mills, early Texas merchant, cotton commission agent, and banker, son of Adam and Janet (Graham) Mills, was born in Todd County, Kentucky, on March 9, 1809. He attended Cumberland College (later Peabody College) in 1826–27. He opened a plantation in Madison Parish, Louisiana, but abandoned it when it was devastated by a flood. In 1830 he moved to Brazoria, Texas, where he joined an older brother, Andrew G. Mills, who had begun trading supplies for cotton and pelts in 1827. As the business expanded, the brothers' burro trains and ships took goods into other parts of Mexico. Bars of Mexican silver were stacked like cordwood in their counting room, and Robert became known as the "duke of Brazoria" because of his possessions.
Robert and Andrew both fought in the battle of Velasco in 1832, and Robert was in the Brazoria County delegation to the Convention of 1833 in San Felipe. He provided use of a schooner to the Republic of Texas in 1836, helped finance the Texas Navy, and supplied blankets, clothing, lead, and other essentials for troops on credit. He married Elizabeth McNeel, daughter of John McNeel, on January 26, 1836, in a bond ceremony. Their $50,000 marriage bonds were indicative of Mills's wealth. Elizabeth died in childbirth in 1837, along with the baby.
After Andrew Mills's death in 1836, the mercantile firm was operated by Robert and his younger brother, David G. Mills. Robert was also associated in merchandising with Samuel B. Brigham in Matagorda County, Theodore Bennett in Brazoria, and John Sutherland Menefee in Jackson County. Robert and David Mills were among the principals of the first insurance company in Texas, incorporated by the legislature in 1837 with a capital stock of $200,000. In 1839 Robert acquired property and built the first cotton compress in Texas at San Luis, west of Galveston island. In 1849 the firm moved to Galveston, and John William Jockusch, Prussian consul at Galveston, was a third partner until 1863. Robert became a partner in Mills, McDowell, and Company of New York and McDowell, Mills, and Company of New Orleans in 1850. His ships transported Texas sugar and cotton to all parts of the world.
Mills was reputed to be one of the wealthiest men in Texas and possibly in the entire South. His large, plantation-style home in Galveston stood on ten acres, later the site of Kempner Park. He was president and director of the Galveston and Brazos Navigation Company, and was associated with the Galveston Wharves. In the absence of banks, between $25,000 and $500,000 in notes of the Northern Bank of Mississippi, countersigned by R. and D. G. Mills, circulated as gold in Texas and New Orleans. Suspension of affiliated firms in New York and New Orleans in 1852 sent "Mills money" below par in Galveston for only one day. While Robert built up the commission and banking business, David operated the plantations, which produced nearly 600 bales of cotton in 1844, probably the largest crop in the republic. In 1852 two of the Millses' plantations produced more sugar than any other in Texas. By 1860 the brothers cultivated approximately 3,300 acres on their four Brazoria County sugar and cotton plantations (Lowwood, Bynum, Palo Alto, and Warren), which also included 100,000 acres of unimproved land. They owned another 100,000 acres scattered over Texas.
At least two of the Millses' steamboats were acquired by the Confederacy for use in the Civil War, and Robert used other vessels for blockade running. He spent most of the war in Europe and Havana. Reputed to have been worth between $3 and $5 million before the war, he was the largest slaveholder in Texas; he freed about 800 slaves in 1865. His firm lost heavily when customers were unable to pay their debts, and suffered additional postwar losses when the cotton market collapsed. In 1873 he declared bankruptcy, but declined to claim homestead law protection. He surrendered his mansion, carriages, and other possessions to pay creditors. In his final years he was dependent on relatives. He died in Galveston on April 13, 1888, and his funeral was held at Trinity Episcopal Church.