Mary O'Hara, née Alsop, was born in Cape May Point, New Jersey to a clergyman's family. Her older sister became the physician and writer Gulielma Fell Alsop. In 1905, Mary married a distant cousin, Kent Kane Parrot, against her father's wishes. After their divorce, she worked in Hollywood as a screenwriter for silent films; her credits included the The Prisoner of Zenda (1922). In 1922, she remarried to Helge Sture-Vasa, born in Sweden, and moved with him to a sheep ranch in Laramie County, Wyoming. The Great Depression ruined the sheep market, so they began breeding horses and running a summer camp for boys. Mary began writing Wyoming ranch stories. Her best-known and loved works were from this period, including the trilogy My Friend Flicka (1941), Thunderhead (1943), and Green Grass of Wyoming (1946). She also wrote a novella, The Catch Colt, and Wyoming Summer, based on her diaries. After Mary and her second husband divorced, she moved back East, and continued to write fiction and nonfiction for children and adults. She also was a talented pianist and composer of piano works and musicals. In 1966, she published an account of writing, composing and producing called A Musical in the Making.