Mary Elizabeth Garrett was born in Baltimore, Maryland. Both of Mary's parents, John W. Garret and Rachel Ann Harrison, came from prominent and wealthy Baltimore families. Mary was the only daughter and youngest child of John W. Garrett.
She was raised in a wealthy household. Although living in a luxurious house in the most prosperous part of Baltimore, Mary Elizabeth had a lonely and unhappy childhood. Her youngest brother was 5 years older than her, and the age difference made it difficult for her to connect with her brothers. She learned about charitable works in her young age as both her parents and grandparents were involved in philanthropy.
Mary went to Miss Kummer's school when she was twelve. At school, she met two lifelong friends, Julia Rebecca Rogers, nicknamed "Dolly" and Elizabeth King, nicknamed "Bessie." Both Dolly and Bessie were from well-known families associated with the Garrett family in Baltimore. Mary was initially excited about school life and enjoyed it, but she gradually got bored because of her school's conservative stances toward girls' education.
Disappointed with the lackluster experiences of school education, Mary quit school at age seventeen and never returned to school in the following years. She preferred to teach herself at home and read literary classics. With only self-education, she learned to speak fluent Italian and French and practiced German and Greek.
Adolescence was not a period of comfort and happiness for Mary. She felt uncomfortable with the Victorian expectations of women at the time and was also uncomfortable with the attitude towards sex in her family. Every family member avoided sex-related topics on purpose, and she had to teach herself about puberty. After leaving school, Mary continued to learn from her father about commerce and the operation of a railroad company, later serving as his secretary.
She helped lay the foundation for the modern American medical education system, and the entrance of young women into it. In 1878, Mary and some like-minded friends founded the "Friday Evening" group (which was to remain active until around 1895) with the vague goal of improving the educational lot of young girls.
Her father's death in 1884 left her and her associates with the means to develop specific projects; in 1885, they founded the Bryn Mawr School for Girls, a college preparatory institution in the Baltimore suburbs, based on the Pennsylvania college of the same name.
The school opened in 1890, and Garrett turned her attention to medical education. Johns Hopkins University (on whose board Mr. Garrett had served) was in the process of building a medical school. At the time, men and women were trained in separate institutions; the men's facilities were, for the most part, of poor quality, little more than trade schools, and those for women fewer, and worse.
Johns Hopkins had a problem; the original endowment was not going to be nearly enough. Mary offered to make up the deficit, but with the stipulations that women would be admitted on the same basis as men, and that all candidates would have to meet certain standards of educational accomplishment.
The conditions were accepted, the first modern American medical school opened in 1893, and Garrett took up the cause of Bryn Mawr College. The Pennsylvania institution was in need of money, which she was willing to supply, again with a condition; that her "Friday Evening" friend M. Carey Thomas be made president.
The trustees were eager to get Thomas, the bargain was concluded, and Bryn Mawr became a world-class facility. Mary then set her sights on the increasingly contentious fight for Women's Suffrage. Poor health was to limit her involvement, though her financial backing continued, and helped insure later success.
Mary was heavily involved in the Women's Suffrage Movement in her adulthood. She hosted the National American Woman Suffrage Association's 1906 convention in her Mount Vernon home. Attendees included Baltimore college women and notable suffragists, like Susan B. Anthony.
She lived her final years at Bryn Mawr, essentially retired from public life after 1909. Mary died at Bryn Mawr College of leukemia at age 61. She was buried in Baltimore’s Green Mount Cemetery, next to her father. She left most of her funds and properties including the Mount Vernon mansion to M. Carey Thomas in her will.