Mary Anne Rymill was baptised on 1 June 1817, the second youngest daughter of William Rymill, carrier, of Banbury, Oxfordshire, England, and his wife, Mary Herbert. She was orphaned when quite young, and went to live with an aunt in London. There she met Susan Pigott, the future wife of the CMS missionary Robert Maunsell. In 1841, hearing of Susan Maunsell's difficulties in coping with four babies at the Waikato Heads mission station, Mary Rymill volunteered to come to her assistance. She arrived at Paihia on the Tomatin on 24 June 1842.
Mary Rymill quickly established herself as 'a very valuable assistant' to the ailing Susan Maunsell. She learned the Maori language and took responsibility for running the household and teaching in the native school. Hazards were an inevitable part of missionary life. Mary Rymill suffered several mishaps when travelling, but they did not prevent her from making many dangerous journeys. While she was at the Orua Bay CMS mission in 1844, assisting Elizabeth Hamlin for a time, a tidal wave swept through the house. Worst of all, on 31 July 1843, the house at Waikato Heads was completely destroyed after a log placed by her on the kitchen fire slipped out into the room. The same night the Maunsells' first daughter was born, and a fortnight later the family suffered yet another fire. The stress of these incidents and subsequent overwork caused a series of illnesses from which Mary Rymill did not fully recover for many years.
Early in 1847 she had to leave Waikato Heads, joining the Chapmans at Rotorua and Maketu, where Thomas Chapman employed her as a teacher. In 1850, however, her health broke down completely: 'Going to Tauranga for a short time for change of air and to meet her old friend Mr Maunsell…she was taken very ill'. Her friends apparently expected her to die, and throughout the 1850s there are constant references in their letters to the state of her health.
She was unable to return to her teaching position, to her distress, and instead went to live with the Browns at the Tauranga CMS mission station in about 1850. She was nurse and companion to Charlotte Brown, her daughter, Celia, and later to Alfred Brown's second wife, Christina Brown. Mary Rymill was treated as one of the Brown family, and she seems to have regarded Charlotte Brown as a mother, although she was about 40 by the time of Charlotte's death in 1855. In July 1863, at the outbreak of war in Waikato, the mission station at Tauranga was temporarily abandoned. Alfred and then Christina Brown returned within the next few months, but although Mary Rymill was extremely anxious to join them again, the mission's difficult situation precluded it.
Mary Rymill had led a useful and fulfilling existence, but a sense of loneliness now overshadowed her life. She had lost touch with most of her own family. Several of the women to whom she had been companion had died, the children had grown up, and suddenly there was no place for an unmarried woman. Her financial position too was precarious; she had some money of her own, and she had been paid by Chapman, but she received nothing else from the CMS.
Her unhappiness and insecurity were ultimately relieved by an invitation from J. W. and Eliza Stack to join them in Kaiapoi. Mary Rymill found 'Her health was so much improved by her visit to Canterbury' that she decided to remain in Rangiora. Her fortunes improved also: in 1863 Alfred Brown arranged to give her £25 a year 'while able to do so'. She thus had 'just enough to get along upon with great care'. As a result of her prudence her estate at the time of her death was valued at nearly £429. She was 'an excellent nurse' and companion to the bishop's wife, Emily Harper, in the 1880s and was also actively involved in the Anglican church. A Mary Anne Rymill Memorial Fund was established in the diocese of Christchurch after her death.
Mary Rymill was 'a short old lady…with hair of…light flaxen colour…small blue eyes, a long, large face with plain features…a quiet dignity of manner…kindliness…warmth and sympathy'. She had 'a keen sense of humour and…agreeable gifts as a conversationalist', being 'trusted alike by old and young'. Her empathy with young people is evident; the Maunsell children remained devoted to her, as did Celia Brown. She also had five godchildren. Her last years were content and secure: she was surrounded by friends, for whom her house was a regular meeting place. Mary Anne Rymill died on 18 December 1897 at Rangiora, 'loved and respected by all who knew her.'