Mary Sadler Powell was born on 14 December 1854 in Methwold, Norfolk, England, the daughter of Mary Sadler and her husband, William Powell, a Wesleyan minister. Mary's father died when she was a child and Mary subsequently lived with her mother in Dorset, Devon and Lancashire. After her mother's death she emigrated to New Zealand to live with her brother at Balclutha. She arrived at the beginning of 1885 and moved to Invercargill with her brother later that year.
In September 1885 Mary Powell began a long association with the New Zealand Women's Christian Temperance Union (WCTU) when she joined the Invercargill branch. She soon became president of the branch and in 1890 attended her first national convention. Under her leadership the Invercargill WCTU grew to be the second largest branch in the country. One of its early projects was starting a home for 'friendless girls', and a maternity ward was soon added.
Powell showed an abiding interest in the education and development of girls. She led a young women's Bible class and, in July 1889, began writing a column called 'Our Girls' for the New Zealand Methodist. Using the pen-name 'Aunt Kate', she aimed to provide the sort of guidance she herself would have liked in her teens when she had longed for someone to show her how to make her life worth something to others as well as to herself. While recommending a disciplined Christian life to her readers, she also supported women's right to preach and vote and advocated the adoption of more rational clothing. The column led to her becoming the confidante of many of her readers. A breakdown in health persuaded her to take a trip to England in 1891, but she continued writing her column for the Methodist.
On her return to New Zealand Mary Powell became involved in the WCTU's national campaign to enfranchise women. 'We have been told we "got the Franchise too easily" ', she wrote later. 'Little do those who make that statement know the miles we walked with that petition, and how women of all ages were interested'. She believed that women's franchise would be 'a tremendous power for good in the land', and worked to get women enrolled as voters.
In the mid 1890s Mary Powell became involved in the organisational side of the New Zealand WCTU, first as corresponding secretary and later as recording secretary and organiser. For about 30 years she remained a key figure in the WCTU's administrative structure. At the beginning of 1900 she travelled to England to attend the biennial convention of the World's Woman's Christian Temperance Union. She also attended the World's Temperance Congress in London, where she was applauded as the first and almost the only woman to take part in the discussions.
Mary Powell was lame as the result of an accident and had to wear a built-up shoe. However, her life as the WCTU organiser was similar to that of an itinerant preacher. She travelled round the country – by steamer, rail, coach, bicycle and on foot – visiting existing branches, starting new ones, and encouraging members to subscribe to the White Ribbon, the WCTU's magazine. In 1905 she reported that during nine months, 'I travelled 2,704 miles, paid nearly 1,000 visits in the interests of the cause, gave 91 addresses, organised six new branches, secured 224 new subscribers to the White Ribbon and about 210 new members for the Union'. On many of her house-to-house visits she rode a bicycle or drove a horse and gig, undeterred by bad weather or poor roads.
Powell was a mainstay of the WCTU, known for her encyclopaedic knowledge of its work in New Zealand. She was remembered by her colleagues as charming and enthusiastic, a gifted speaker, a born organiser and an 'earnest, self-sacrificing worker'. The temperance cause was her main interest, but she loyally supported the broad range of work undertaken by the WCTU. In 1919, when she made her final report as organiser, she was made a life member of the union.
Mary Powell also worked as an organiser for the prohibitionist New Zealand Alliance during licensing-poll election campaigns. She never married, and on her retirement from active work for the WCTU settled in Dunedin, where she was cared for by her niece. She died in Dunedin on 8 March 1946.