At Brookings, she met Herbert Walter "Walt" Hargreaves, who was finishing up his Ph.D. in economics from Duke University. They married on August 24, 1940, and she followed her husband when he was hired to teach at what is now the University of Texas at El Paso. Walt Hargreaves joined up during World War II, and they moved to various posts for his training. He served also as a member of the economic reconstruction delegation to Germany, and for most of the war she lived in Brooklyn, New York. Soon after the war, Walt Hargreaves was hired as a professor of economics at the
University of Kentucky in
Lexington. When they arrived in Kentucky, Mary Wilma Hargreaves met
Thomas D. Clark in the History Department who invited her to guest lecture in his classes on American land policy, the subject of her dissertation. Finally, in 1951, Mary Wilma Hargreaves earned her Ph.D. in history from Harvard University. She applied for a job in the university's History Department, but instead was hired as a typist for James F. Hopkins who had been working on gathering the letters and works of
Henry Clay for a bound series. By 1957 with a grant from the Eli
Lilly Endowment and sponsored as part of a program by the National Archives and Records Division, Hopkins was appointed editor-in-chief of the Henry Clay Papers and Hargreaves was hired as associate editor. Together they edited the first five volumes of the series. ==Career as historian==