Morriss began her career at Mount Holyoke College, teaching American history. While there, she was active in the local chapter of the College Equal Suffrage League.[5] In 1923, she was hired by Brown University to serve a professor of history and as Dean of Women,[6] a post she held until she retired in 1950. She more than doubled the number of women enrolled at Brown during her tenure, and saw the 1927 creation of a Women's Building on campus.[7] Her name was sometimes lampooned as "Peggy Push" (from "Margaret Shove"), to emphasize her assertive style of leadership. In 1951, Brown established a Margaret S. Morriss Scholarship in her honor.[2] A dormitory built in 1960 was named for Morriss.[8]
From 1917 to 1920, during World War I, she joined the Young Women's Christian Association efforts in France, Germany, and New York,[9] as director of recreation for nurses.[10] During World War II, she was president of the American Association of University Women (AAUW) (1937-1941)[11] and president of the New England Association of Schools and Colleges (1941). In 1965 the Connecticut chapter of the AAUW established an international fellowship in her name.[2]
