She founded Bryn Mawr School, a private college-preparatory school for girls in Baltimore, and generously donated to Bryn Mawr College of Pennsylvania with the requirement that her intimate friend Martha Carey Thomas be the president. Like many other suffragists of the nineteenth century, Garrett chose not to marry; instead, she kept a lifelong working and emotional relationship with Thomas. In her later years, she collaborated with her longtime friends Susan B. Anthony and Anna Howard Shaw to try to secure the right for women to vote in the United States.
