Robert Langton Douglas was born in England, the son of a Reverand. He himself was ordained a priest and served in the church from 1888 until 1900, when he left to pursue a career in history, art history and art criticism. He held a wide variety of positions in these capacities, including as a professor in Australia and as a writer and lecturer on art. In 1904 he became an art dealer, although this did not preclude his later serving as the director of the National Gallery of Ireland between 1916-1923. His specialty was Sienese painting. Among his important clients were the famous collector J.P. Morgan and the director of Berlin's Kaiser Friedrich Museum, Wilhelm von Bode.
Douglas' personal life was equally varied. He married three times, first in 1891 to Margaret Jane Cannon, with whom he had five children; second in 1902 to Gwendolyn Henchman, with whom he had three children, and lastly to Jean Stewart in 1928, with whom he had two children. In addition he had three children with Grace Hutchison. Douglas died in Fiesole, Italy, and is buried in Siena.