Rosemary Theby was born Rose Masing in St. Louis, MO, on April 8, 1892. She began her career at age 27 in 1912, with a part in The Godmother (1912), and went on to appear in more than 90 films. She was much in demand and was one of the most sought-after character actresses, and survived the transition from the silent era into the "talkie" stage. She was married to director Harry Myers for more than 20 years. He died in 1938 and two years later, after appearing in Kayıp Dünya (1940), she retired to Kansas City, MO.

She died June 10, 1973, at the Virgil Convalescent Center in Los Angeles, CA.