Raleigh Colston Thomas was born on October 8, 1844, in Baltimore, Maryland, to John and Annie Thomas. His father was a physician and bank president who owned $100,000 of real estate and $300,000 of personal property by 1860. Thomas grew up and attended school in Baltimore, and he attended the University of Maryland's preparatory department in the late 1850s.
He sided with the Confederacy during the Civil War, and he mustered in as a private in Company C of the 1st Maryland Cavalry in 1863. He served until the end of the war.
He returned to Baltimore after the war, and he married Mary McDonald on November 25, 1868. They had at least three children: Mary, born around 1870; Raleigh, born around 1876; and Reginald, born around 1879. He worked as a merchant, and by 1870, he owned $350,000 of real estate and $40,000 of personal property. He also employed at least two white and two Black domestic servants. He and his family travelled to Europe in 1872. His passport application described him as 5 feet, 11 ½ inches tall, with dark brown hair and dark brown eyes. He died in January 1888.