Annabella Mary (known as Mary) Webster was born on 19 May 1864 at Mangungu on the Hokianga Harbour, one of twins born to William Webster, an interpreter and sawmiller, and his wife, Annabella Gillies, whose mother was of Ngati Toro of Ngāpuhi. Her parents met when Annabella Gillies was living at the Mangungu mission run by Jane and John Hobbs. Mary and her twin brother, Alexander, attended a mission school, and Mary was sent to a private girls' school in Auckland to complete her education. However, their mother's first language was Māori, so they grew up in a bilingual household.
On 28 April 1886 at Devonport, Mary married John McKail Geddes, a wealthy importer and merchant, 21 years her senior. They were to have seven children, one of whom died aged eight. The family lived in comfort in a mansion, Hazelbank, in Wynyard Street, and there were regular excursions to resorts and ski-fields abroad. But Mary Geddes was not content with her luxurious lifestyle. She was intensely concerned about the welfare of women and girls, and although she did not question that the paramount role of women was as mothers, she believed that they should develop a strong public voice to further their concerns. A friend of Frederic Truby King, she was a foundation committee member of the Auckland branch of the Society for the Promotion of the Health of Women and Children (later known as the Plunket Society), formed in 1908. She also served on the Auckland committee of the New Zealand Society for the Protection of Women and Children for 13 years.
Mary Geddes remained bilingual throughout her life and maintained her links with Hokianga. By the early years of the century she was running an informal domestic training school for Māori in her home, which was held up as a model by Lady Stout. Her pupils were the daughters of her Hokianga relatives whom she then placed in the households of friends. The scheme addressed the shortage of domestic labour, but it was also aimed at helping Māori and Pākehā understand one another.
When John McKail Geddes died in 1910, Mary was left a young widow of considerable means. Her social status, connections with business people and personal qualities enabled her to take a leadership role among women in Auckland. It was unusual at this time for women of Māori ancestry to involve themselves in Pākehā organisations, but Mary Geddes was at home in both worlds. During the First World War she was a board member of the Auckland Provincial Patriotic and War Relief Association. After the influenza epidemic struck in 1918, she established community kitchens and nursing and medical relief services.
She was on the board of the Auckland Young Women's Christian Association from 1906 to 1925 and was president from 1913 to 1919, the period in which the association underwent its greatest expansion. While travelling overseas Mary Geddes had admired impressive YWCA buildings, and she returned with a vision of such a building for Auckland. She spear-headed a campaign that raised the large sum of money needed, and in October 1918 opened imposing new Queen Street premises designed by the eminent architect William Gummer.
Mary Geddes's eldest daughter, Mary, followed her mother into the YWCA, establishing the Hearth Fire Movement and in 1915 taking up the Sydney-based post of secretary of the girls' work department for Australia and New Zealand. In 1920 mother and daughter were on the New Zealand Field Committee, which preceded the establishment of a national association separate from Australia, and young Mary was one of the first two New Zealand delegates to the World's YWCA.
Mary Geddes, along with other YWCA representatives, took a leading role in the revival of the National Council of Women of New Zealand in Auckland in 1917. Revealing her feminist sympathies, she spoke at public meetings and in 1919 was part of a deputation that visited Sir Joseph Ward to urge the passing of a law to enable women to stand for Parliament. A staunch Presbyterian, Geddes often spoke most strongly about moral issues, such as the age of consent. In 1918 she drew the attention of the Auckland branch of the NCW to the practice in some drapers' establishments of dressing mannequins in shop windows in front of the public. On the other hand she urged the YWCA to accept dancing, a source of contention in the Christian organisation, and supported sex education lectures for girls.
By the mid 1920s Geddes had withdrawn from active work in women's societies, and the church became her main outside focus. The family business failed during the depression of the 1930s, and in 1939 Hazelbank was sold to Auckland University College, later being developed as the Elam School of Art. Mary Geddes, her daughter Mary and a friend, Wilhelmina Keller, moved to Remuera. She died at her home on 5 December 1955, aged 91 years, survived by two daughters and two sons.