Mills was a Texas merchant and banker. He was born in Kentucky in 1809 and attended Peabody College. He moved to Brazoria in 1830 and opened a business with his brother Andrew, trading various goods and running a shipping business. The Mills brothers amassed great wealth in their business and used their success to help finance the Texas Revolution, including the Texas Navy. Andrew passed away in 1837 and Robert partnered with his younger brother David to continue the business. In addition to trade, they started the first insurance company in the Republic. They moved their business to Galveston in 1849, and Robert partnered with Mills, McDowell and Company of New York and McDowell, Mills and Company of New Orleans. They became the largest cotton and sugarcane producers and traders in Texas, and at the height of their business they were the richest men in Texas. During the Civil War, some of their steamboats were used by the Confederates as blockade runners. In 1873, Mills declared bankruptcy and he lost most of his wealth and possessions, relying on family for his last years. His Galveston homestead is now the site of Kempner Park. He died in 1888.