Margaret Casey was born in 1903 in the Georgetown section of Washington, D.C. She studied art in the studio of Bertha Perry, and from 1924 to 1926 at the Corcoran Art School. She later studied under Henry Varnum Poor at the Colorado Springs Fine Arts Center.
After working as a commercial artist from 1928-1929, Casey began studying at the Phillips Memorial Gallery in 1931 under C. Law Watkins. There, she met her husband, painter Robert Franklin Gates, and married on January 7, 1933. Robert Franklin Gates (1906-1982), who came to Washington, D.C. in 1930, also studied at the Phillips Gallery Art School and worked with Karl Knaths between 1934 and 1947.
Between 1934 and 1941, Robert Gates, with other artists including Mitchell Jamieson and Prentiss Taylor, made several painting trips to the Virgin Islands on a fine arts commission from the U. S. Treasury Department. Margaret Gates accompanied her husband and produced artwork of her own. She also documented their travels. During this same time, Robert taught at the Phillips Gallery Art School and Margaret was employed as the Art School's secretary. In 1937, they purchased a house in McLean, Virginia where Margaret lived until 1980.
In 1939, Margaret Gates won honorable mention in a national mural competition held by the Section of Fine Arts of the U. S. Public Buildings Administration and was subsequently commissioned by the Federal Works Agency to execute a mural for the Post office at Mebane, North Carolina. The mural was completed and installed in 1941.
During the late 1940s and early 1950s, Margaret Gates wrote articles on art for Washington, D. C. publications including the column "The People vs. Art" for American University's Right Angle, and for the magazine The Washington Spectator.
Margaret and Robert Gates were divorced circa 1955. They had no children.
Margaret Gates exhibited her work in the first exhibition at the National Gallery of Art in 1941, as one of many winners of a national competition sponsored by the Federal Works Agency for artwork to decorate a Marine hospital for lepers in Louisiana. She also exhibited in the "Group Show of Washington Painters" at the Bignou Gallery in New York City, as well as at the Corcoran Gallery of Art, the Philips Collection, the Virginia Museum of Fine Arts, the Cincinnati Art Museum, and the Baltimore Museum of Art. A retrospective of her work was exhibited at the Watkins Gallery of American University in 1981. Several of her paintings are in the collection of the Phillips Gallery.
Margaret Casey Gates died on November 4, 1989, in Mitchellville, Maryland.