Mahon was a member of both the Gaelic League and the Irish Women's Franchise League. While she was a non militant member she expressed support in The Irish Citizen. In 1906 Mahon began her involvement with the teachers trade union and joined the Birr Association and made a speech about equal pay at the Irish National Teachers Organisation annual congress. She then pointed out the lack of women on the INTO executive. Arguing the point she put herself forward for consideration for the position of Vice-President of the association in March 1907 and, although she wasn't elected, the question was so public that the INTO created new positions on the executive for women and offered one to Mahon.[1][4][3]
In the first years of her involvement Mahon was successful in a number of her goals, she increased recruitment and prevented women teachers being forced to cover Laundry and Cookery as subjects. When the Birrell grant was adopted in the summer of 1908 Mahon pointed out that the principal of equal pay had been established, as the grant was distributed to all teachers. Thus she claimed the precedent was made.[1][3]
In April 1911 she was elected to the position of Vice-President unopposed. That year her goal was to prevent teachers on maternity leave to be required to provide their substitute at their own expense. In 1912 Mahon was elected the first woman president of the INTO. Almost immediately there was a crisis with the dismissal of the vice president Edmond Mansfield and issues between the INTO and the board of education. Mahon was sent to London to request the chief secretary Augustine Birrell to hold an inquiry. Although the secretary was biased against her position, he admitted her statement presented the case well and the Dill commission was set up.[1][4][3][5][6]
Mahon was re-elected in 1913 despite he usual one term assumption. The teachers’ paper, Irish School Weekly in 14 December 1912 claimed "the general who has in every encounter routed the enemy's cavalry, must be retained at the head of the forces." This allowed her to give evidence as president to the Dill Commission which went on to vindicate the teachers and recommend changes but refused to recommend the reinstatement of Mansfield. As a result, the following year she resigned from the executive. She stepped down Easter 1916. She was questioned for her vocal support of the Easter rising.[1][4][3][7]
An issue which arose in her final year on the executive was the War bonus, which granted men double the pay of women. Mahon protested this and declared success in November 1916. In 1919 she criticized the Education bill by Ian MacPherson and called the INTO executive British apologists. She made similar accusations against INTO's president and the general secretary T. J. O'Connell. As a result in 1920 she was charged with libel. Mahon refused to accept the British court's jurisdiction but once she was silenced the INTO dropped the case. She stayed out of public life on a national level from then on.[1][4][3]
Another woman was not elected to be president of the INTO until Kathleen Clarke in 1945.[1]