O'Toole studied at Washington College of Law, completing her Bachelor of Laws in 1908, and Master of Laws in 1914. She was first appointed Judge of the Municipal Court of Washington, D.C. by President Harding in 1921. At the time of her appointment, there were only three woman judges in the United States. In 1924, she was a delegate at a meeting of the American Bar Association, held in London. She was re-appointed to the Municipal Court by President Coolidge in 1925. O'Toole was a member of the board of trustees of the Washington College of Law, and held the position of Acting Dean of the College while Dean Grace Hays Riley attended the American Legion Convention in Paris in September 1927. In January 1928, E. E. Dudding, national president of the Prisoner's Aid Society, wrote to the Department of Justice nominating O'Toole to fill the vacancy on the bench of the District of Columbia Supreme Court caused by the resignation of Judge Adolph Hoehling, stating, "There is no better lawyer in Washington or anywhere else. She knows the law. She is judicial. She would make one of the best judges to be found and would, I think, raise public opinion of Federal courts". O'Toole was reappointed to the Municipal Court by President Hoover in 1929. In 1930, O'Toole was named one of the 50 women who had done the most for Washington by The Washington Post. Referring to O'Toole as a "pioneering woman member of the bench" and "a fixture in Washington's judiciary system", The Washington Post in 1934 reported on how, prior to her initial appointment, "the very mention of her name as a prospective member of the bench made usually dignified men pace the floor and use undignified language". In 1931, she was elected to the executive committee of the National Association of Women Lawyers, which her colleague Dr Ellen Spencer Mussey had helped found in 1919. She spoke at the Thirty-Seventh Annual Meeting of the National Association of Women Lawyers on 5 August 1936. O'Toole was also a member of the Phi Delta Delta Women's Legal Fraternity and the District of Columbia Women's Bar Association.
Views
Capital punishment
In September 1929, The Washington Daily News reported that O'Toole was "one of the most ardent anti-capital punishment workers in the District ". She is stated to have said, "I am convinced that experience, history, and psychology teach us that the death penalty is wrong. In any event, it does not suit our times and the upward and onward progress of our civilisation. That we constantly try to mitigate its horrors for ourselves by seeking and selecting the least painful methods of inflicting it, by limiting the number of witnesses, and so on, is the sure argument for its abolishment". O'Toole led a successful campaign to have the Washington Chamber of Commerce go on record as opposing capital punishment.
An interview with O'Toole was published in The Washington Post on February 26, 1928, under the headline "High Divorce Rate a Sign of Progress: Some Very Frank Opinions From a Woman on the Bench, Judge Mary O'Toole, of the Municipal Court of the District of Columbia, and Why She Does Not 'View With Alarm' Recent Increase in Marriage Breaks". She was quoted in the interview as saying, "A woman seeks divorce for one of three main reasons: To protect herself from a person no longer tolerable; to be free, perhaps to acquire other bonds; or to free her husband, her economic independence lulling the fear of want and poverty, so large a factor in the life of women of an earlier day".
Women's suffrage
O'Toole campaigned for women's suffrage in Washington, D.C., and wrote a column for a local newspaper that started, "Why do I favour suffrage for the District? As well ask me why I want to eat. One seems quite as obvious as the other".
Other interests
O'Toole was the first president of the Women's City Club of Washington, D.C. which was founded in 1919. On the club's ethos, O'Toole said the group hoped to attract not only women in the academic and professional sphere, but housewives as well. The Club's charter called for a "better understanding among women, and a place for women to meet and socialise". She was the first woman to be chosen as director of the Washington Chamber of Commerce, and was a director in the Citizens Savings Bank of Washington, D.C. O'Toole also had membership with the Sulgrave Institution, the Columbia Historical Society, the Catholic Actors Guild, the American Catholic History Association, the American Association of University Women, and the National Women's Republican Club.
