Mabel Vernon was born in Wilmington, Delaware. Her father was editor and publisher of the Wilmington Daily Republican . Part of a large Quaker-Presbyterian family, she went to Swarthmore with Alice Paul and graduated in 1906. During her college career she won awards as a debater. Vernon taught Latin and German in a Pennsylvania high
school before attending a National American Woman Suffrage Association (NAWSA) conference in Philadelphia in 1912. She later returned to school and earned a master's degree in political science from Columbia University
At Alice Paul's invitation, Vernon worked as a regional fund-raiser and recruiter for the Congressional Union for Woman Suffrage (CU) shortly after its formal organization in 1913. The following year she led the CU campaign against Democratic congressional candidates in Nevada along with Anne Martin. She soon headed the push to establish state branches in several
western states. When the CU asked Sara Bard Field and other suffrage envoys to travel cross-country by automobile in 1915, Vernon worked as the advance person, organizing events and meetings in several major cities. She joined Alice Paul and others in testifying for woman suffrage before the House Judiciary Committee at the end of that year.
Vernon has been described by her fellow activists as the first, and perhaps the most outstanding, of NWP organizers. She was named secretary of the newly formed NWP in June 1916. The following autumn, Vernon worked as a regional organizer, doing street speaking and holding rallies to encourage citizens not to support the reelection of legislators opposed to a federal suffrage amendment. She
participated in the 1919 “Prison Special” tour, which did much to dispel popular fears of NWP militancy and win sympathy for the sacrifices that NWP activists had made for the suffrage cause. During the two years leading up to the ratification of the 19th Amendment, Vernon reprised her role as a regional organizer, working especially in Georgia, Kentucky, and Delaware.
Mabel Vernon was also notable for her audacious demonstrations during major presidential addresses–calling out to President Wilson during his Independence Day speech in 1916. After Wilson's closely contested reelection in November 1916, she and other NWP activists secured front-row gallery seats for his annual address to Congress. During the speech, Vernon and the others unfurled a suffrage
banner from inside Vernon's coat, an action that won publicity across the country. Vernon was also among the first group of NWP women sentenced to brief terms in the District jail when she was charged with obstructing traffic while picketing the White House in June 1917.
Vernon remained active in the NWP in the 1920s and served as its executive secretary. Deeply involved in the Women for Congress campaign, she participated in a 1926 transcontinental motor trip that encouraged support for female candidates for office. Vernon also worked for the Equal Rights Amendment. In 1930 she shifted her activism to the Women's International League for Peace and Freedom.
From the 1930s through the 1950s she devoted her time to the issues of disarmament, peace, Latin American rights, and international relations. She shared a Washington, D.C., residence with her close companion and fellow activist Consuelo Reyes-Calderon, from 1951 to 1975.