Rachelina Hepburn Stewart was born on 22 April 1873 at Dunedin, New Zealand, to Rachel Hepburn and her husband, William Downie Stewart, a barrister. She displayed a strength of character and sense of determination from an early age. After her mother's death in 1878 and, in 1881, her father's remarriage and entry into politics, she provided maternal support for her three younger siblings. This drew the children into a close-knit unit upon which they each came to depend. They saw little of their parents and were 'ruled by a severe nurse'.
Rachel (a name she would insist on in her adult years) attended a private primary school in Dunedin. From 1885 to 1892 she went to Otago Girls' High School, where she excelled academically. On leaving, she wished to pursue a career in medicine, but her father believed this would be too strenuous an undertaking. Instead, after attending a Swiss school to improve her French, in 1893 she went to Somerville College, Oxford, to study modern history. In 1896 she became the first New Zealand woman to complete the BA course at the University of Oxford.
Her father supported her decision to remain in Britain and join the Women's University Settlements Scheme, which sought to improve the education, welfare, physical environment and self-reliance of working-class women and children in South London. Her contact with leading British social reformers such as Octavia Hill, who taught her estate management, and the many skills she gained from counselling, office administration and budget management prepared her for the life she would lead in New Zealand. She returned in early 1899 following the death of her father.
An agreement was reached with her siblings that she would keep house for her brother William Downie Stewart and provide him with the support he required to complete his legal studies at the University of Otago. Friction with their stepmother was resolved when she granted permission for Rachel and her brother to remain in the Dunedin family home, Ashentree, after she departed for England.
While visiting her brother George at Crookston, South Otago, Rachel met George Whitefield Armitage, an accountant for the local branch of the Bank of New Zealand. In 1902, while sailing to England with her sister Mary, Rachel received a proposal by cable from George Armitage. Their marriage on 4 March 1903 at Crookston Presbyterian Church was the social event of the district. The couple settled on a small property, Garmancare, in Temuka, where their two sons were born.
Both Rachel and George Armitage entered into the life of the district. Rachel formed, and was president of, the St Peter's Anglican Church Ladies' Guild which, under her skilled leadership, raised substantial funds to repay the debt owing on the church buildings. She also established a branch of the New Zealand Federation of Women's Institutes in Temuka, and maintained a correspondence with her colleagues in England.
Besides assisting George Armitage in managing the day-to-day routines of the farmlet, she supervised the running of her home and extended hospitality to many church dignitaries and important visitors to the district, including Dr Truby King and his wife, Isabella. Truby was a family friend, and she became committed to his ideas and practices, determinedly setting about establishing the headquarters in Timaru of the Society for the Health of Women and Children (Plunket Society) in South Canterbury. The isolation of country women and their lack of access to Plunket facilities concerned not only the society but also the Women's Institute, and during 1909, under her guidance, branches were formed in outlying districts. Rachel Armitage was president of the Temuka branch of the Plunket Society from 1914 to 1928 and a member of the central council in 1928. Later, she continued to support its activities through fund-raising, gathering clothes and visiting mothers. She was still an energetic member in 1953.
Throughout her married life Rachel Armitage was a confidante of and support for her sister Mary and brother William Downie Stewart, providing him with a refuge from his busy political career, in which Mary acted as his hostess. After the death of her husband, George, in 1943, she dedicated more time to assisting Mary in the care of her brother, who was severely incapacitated by rheumatoid arthritis. She died on 14 May 1955 during a stay at the family home in Dunedin; she was survived by her two sons.