In 1915 Marsh made her first film appearance, an uncredited one, in the short The Mad Maid of the Forest. Later that same year she was also cast in Hearts Aflame and then billed as Dorothy Rosher. In 1917 she appeared too in The Little Princess and in no less than five other productions in 1918, including the comedy-drama Women's Weapons for Paramount Pictures. After these minor roles as a baby and toddler, Marsh finally became a star in Mary Pickford films such as Daddy-Long-Legs (1919) and Pollyanna (1920).

Marsh made her last film appearance as a child in 1921 but returned to films nine years later with a role in King of Jazz, in which she sang with Bing Crosby. She subsequently worked in a series of shorts and other feature films before she played W. C. Fields's daughter in You're Telling Me! in 1934. She then continued performing on screen in small roles for the next decade. In 1936, on radio, she sang on the CBS program Flying Red Horse Tavern. The following year, during the filming of Charlie Chan on Broadway, Marsh met and later married Writer Charles Belden, who had co-written that film's screenplay. Their marriage ended in divorce in 1943 -- first in Los Angeles, California, on August 26, 1943, followed by a second divorce October 23, 1943, "so she won't have to wait a year before remarrying." Marsh made her final film appearance the next year in Follow the Leader.

Marsh later managed a stationery shop, and she died at age 87 in Ojai, California in 2000.