Marion Greenwood received a scholarship to the Art Students League in New York where she studied with John Sloan, George Bridgman and Frank Vincent DuMond. Greenwood also studied lithography with Emil Ganso and mosaic with Alexander Archipenko. In 1932, she traveled to Mexico where she met artist Pablo O'Higgins, who urged her to try her hand at fresco painting. Her first fresco was on the stair well at the Hotel Taxqueña and she then completed a seven hundred square foot fresco at the University of San Nicolas Hidalgo in Morelia. Greenwood also worked with Diego Rivera on fresco murals at the Mercado Rodriguez and was the first woman to receive a mural commission from a foreign government.

Upon her return to New York in 1936, she joined the ranks of artists working on the WPA Federal Arts Projects. During WWII, Greenwood was one of two women artists appointed as artist war correspondents. After the war, she focused on lithography and easel paintings and her first solo exhibition was held in December 1947. A retrospective of her work was mounted at Woodstock Art Association in 1972.

Greenwood was a visiting Professor of Fine Arts at the University of Tennessee in 1954 and was Artist-in-Residence at Syracuse University in 1965. She was a member of the National Association of Women Artists, Audubon Artists, Mural Painters, American Artists Congress, Mural Painters Guild, Woodstock Art Association, and was elected in 1959 to the status of National Academician in the National Academy of Design.

She was awarded the Lithography Prize from the John Herron Art Institute, the Lippincott Figure Prize from the Pennsylvania Academy of Fine Arts, the Altmann Figure Prize at the National Academy of Design, the Grumbacher Prize, and the Lillian Cotton Award from the National Academy of Design. Greenwood's work is represented in the Library of Congress, the Metropolitan Museum, the Museum of Modern Art, the Bibliotheque nationale de Paris, the Carnegie Museum of Art, the Pennsylvania Academy of Fine Arts, the Tel Aviv Museum, the Butler Art Institute, the Mint Museum, the Montclair Art Museum, the Norfolk Museum, the National Academy of Design, the Indianapolis Museum of Art, the Smithsonian American Art Museum, and the Museum of Fine Arts Boston.