Born in Philadelphia in 1822, Mary was the second child of her father, Thomas, and her mother, Mary Ann, for whom she was named. Some historic references list her as Mary Ann M'Clintock Jr. From an early age, Mary was a participant in her family’s reform activities.
As the daughter of Quaker activists, Mary, like her older sister Elizabeth, took part in Anti-Slavery, Temperance, and Woman’s Rights events. Her signature can be found on anti-slavery petitions dating to her teenage years, demonstrating an early interest in movements for equality.
The Report of the Woman’s Rights Convention indicates that Mary, like her mother, her father, and sister, took an active part in the proceedings. In addition to seating attendees, circulating the Declaration of Sentiments for signatures, and signing her name to the Declaration, Mary served as secretary and kept a clear record of the two days’ proceedings. On the last evening, July 20th, Mary delivered an address to all those present. According to the convention minutes it was “a short but impressive address, calling upon woman to arouse from her lethargy and be true to herself and her God.”