Mary Ann Recknall McCarthy was born in Dunedin, New Zealand, on 11 August 1866, the fifth of eight children of Elizabeth Ann Pyke and her husband, Samuel McCarthy, a locksmith. Mary (or Polly, as the family knew her) became a pupil-teacher at the Normal School in Dunedin about 1884, entered Dunedin Training College in 1887, and from 1888 to 1902 taught as an assistant at Naseby in Central Otago. She returned to Dunedin, again to the Normal School, for 1903–4, and then from 1905 to 1913 took sole charge of the Waihao Downs School, south of Timaru. She was advanced in her views on health; a young niece, sent to live with her, was required to take cold showers, and ate healthy, carotene-rich puddings made using boiled seaweed.
Mary McCarthy's political career began with the New Zealand Women's Christian Temperance Union. At the 1906 national convention she was elected superintendent for influencing the press. In 1907 she was a member of a delegation to the minister of education, George Fowlds, on 'scientific teaching' (sex education) in schools, and was elected national treasurer. In this capacity, as in most offices she held, she was vigorous. She chaired the Waimate–Waihao Downs branch, and spoke at their celebration in 1908 of the 15th anniversary of women's suffrage. Between 1906 and 1911 she attended every national convention, putting forward motions or arguing on education, equal pay and economic equality in marriage, raising the age of protection, and the effective propagation of the WCTU's platforms. She published articles on the British, Indian, Australian and New Zealand contagious diseases acts in the White Ribbon. Her attendance then ceased for some years, in part because of the pressures of work, until 1916, when she was elected national superintendent for purity and moral education.
Local problems and possibly ill health prompted her to consider early retirement from teaching. Because she expelled the children of the owner of Waihao Downs station, Edward Richards, she angered influential members of the community, who in 1909 requested that she be replaced by a man. This was a common practice when the rolls of sole-charge schools taught by women swelled to justify a second teacher. She stayed only long enough to gain her pension before retiring in 1913.
Mary McCarthy devoted the rest of her life to political activity. By 1917 she had formed the Dunedin branch of the Women's International League, and appears to have run it almost singlehandedly for its short life. She also agitated on behalf of conscientious objectors. In mid 1919 she left for England and Europe. She attended the formation of the Guild of Builders in England and represented the National Peace Council of New Zealand and the New Zealand Section of the Women's International League for Peace and Freedom at the International Congress of Women in Zurich.
After her return to Dunedin in 1920 she corresponded and worked closely with the Auckland branch of the WILPF. She was the link between the women's organisations, WCTU and WILPF (which later strengthened its ties with the labour movement), and the labour newspaper, the Maoriland Worker. The WILPF's aims – 'true internationalism, anti-militarism and the interests of working class women' – by now corresponded closely with her own.
She joined the New Zealand Labour Party after its inception in 1916, no doubt influenced by her brother Arthur McCarthy, who played an important role in its formation. She toured New Zealand for the Labour Party in the early 1920s, speaking and helping to establish new branches in country areas. She spoke to the conference of the Otago branch of the New Zealand Educational Institute on the control of industry in 1921, advocating guild socialism. When a commentator noted slightingly that guild socialism was being tried in Russia, her sharp response was that 'There was a good deal to be learnt about Russia, and if they thought it was a country to be laughed at they were making a big mistake'.
The main concern of her last years was internationalism. Active in the local Esperanto club, she corresponded internationally with Esperanto enthusiasts, especially those in Russia. She never married, and died in Dunedin on 13 October 1933. Her sister, Emma McCarthy, wrote of her that as a teacher she was able to find 'the particular bent of many pupils, which led to profitable employment for them'; and that she was 'an idealist, and like all such, intensely practical'.