In 1914, at the age of 56, Cooper began courses for her doctoral degree at Columbia University, but was forced to interrupt her studies in 1915 when she adopted her late half-brother's five children upon their mother's death. Later on she transferred her credits to the University of Paris-Sorbonne, which did not accept her Columbia thesis, an edition of "Le Pèlerinage de Charlemagne". Over a decade she researched and composed her dissertation, completing her coursework in 1924. Cooper defended her thesis "The Attitude of France on the Question of Slavery Between 1789 and 1848" in 1925. Cooper’s retirement from Washington Colored High School in 1930 was by no means the end of her political activism. The same year she retired, she accepted the position of president at Frelinghuysen University, a school founded to provide classes for DC residents lacking access to higher education. Cooper worked for Frelinghuysen for twenty years, first as president and then as registrar, and left the school only a decade before she passed away in 1964 at the age of 105. At the age of 65, she became the fourth Black woman in American history to earn a Doctor of Philosophy degree. Her work was eventually published in an anthology of medieval French literature, and was requested for classes and the bookstore at Harvard.