On August 18, 1920, Mahony registered to vote in Boston after suffrage was achieved.
Mary Eliza Mahony registered to vote in Boston’s Ward 13 on August 18, 1920, the day the Nineteenth Amendment was ratified following Tennessee's historic vote. This action positioned her among the first women in Massachusetts to exercise this newfound right. Mahony's participation was an early assertion of her civic engagement and political influence at a time when women across the country were beginning to claim a place in the democratic process. Her involvement was timely and significant, symbolizing the powerful nexus between her earlier struggles for racial and professional equality and the larger women's rights movement. This event highlighted her enduring legacy in both nursing and civil rights activism, promoting a vision for equality that transcended gender and race.