Mary was born into a Quaker family on March 8th, 1856 in Richmond, Indiana. She was the daughter of Jonathan Wright Plummer (a wholesale druggist) and Hannah Ann (Ballard) Plummer. Mary graduated from Friends Academy and studied for a year at Wellesley College before she began teaching in Chicago. She was an avaricious reader and fluent in German, French, Spanish and Italian. She wrote many poems that were published in the Atlantic Monthly and Scribners.
She graduated in 1888 from the first class of the first established library school, “the first class in library science on the planet” the Library School of Columbia College where she was a student of Melvil Dewey’s and had excelled. It was a class of 17 women and 3 men. She was a cataloguer at the St. Louis Public Library and then later moved to the Pratt Institute, where she created a program in library studies. She spent nine years as the director of that library and is credited as being the one to create a separate room that was dedicated entirely to the children’s collection. She is also credited with originating the idea of initiating special training for children’s librarians. Upon retiring as director of the Pratt Institute Free Library, she moved to New York and the New York Public Library where she founded another library training program. Mary is credited with originating and promoting the idea of ethics for the library profession.
Over the course of her career, she wrote several children’s books, authored articles in librarianship, published essays in literary magazines, and she held various positions in the American Library Association (ALA). Mary held leadership roles in the library profession at a time in history when it was quite out of the ordinary for women to lead. She served as president of the New York State Library Association, the New York Literary Club, the Long Island Literary Club and she was elected President of the ALA in 1915.