Nettie Florence Armstrong was born probably in Carthage, Missouri, USA, on 18 March 1875, the daughter of Frances Haxton and her husband, James Armstrong, a timber merchant. As a child, Florence lived in Arkansas, but around 1885 moved with her family by covered wagon to Washington state. Florence found companions among Native American children and became a good horse-rider. She attended Walla Walla College, which her father had helped build, and was a pupil in the first class to matriculate.
By the time Florence Armstrong reached adulthood she had joined the Seventh-day Adventist church. Adventists were progressive in their attitudes towards women, and their conception of religion encompassed the health of the body as well as that of the soul. Armstrong studied nursing at Battle Creek Sanitarium, Michigan, and worked at the Spokane Sanitarium in Washington. Around 1895 she enrolled at the American Medical Missionary College in Chicago, having been encouraged to pursue a career in medicine by Adventist health reformer John Harvey Kellogg. Trained as an allopath, she graduated with a college degree and state diploma in medicine in 1900.
Florence Armstrong arrived in New Zealand as a medical missionary around August 1901, reputedly the first woman doctor to be sent overseas by the Adventist church. On 13 August 1901 at Papanui, Christchurch, she married Peter Martin Keller, an Adventist doctor, whom she had met as a student in Chicago. The couple took charge of the Christchurch Medical and Surgical Sanitarium, the only Adventist health clinic in New Zealand. Located at Papanui, it could accommodate 20 patients suffering from chronic and non-contagious illnesses, and also handled outpatients. In her treatments, Florence Keller used the latest hydrotherapeutic and electrical equipment, such as thermal, vapour and 'electric light' baths, and 'nebulisers, atomisers, and hydraulic air compressure apparatus'.
Keller placed a strong emphasis on living healthily. She warned patients of the dangers of smoking cigarettes and drinking alcohol, tea and coffee, and advocated plenty of rest, exercise and fresh air. A vegetarian and keen cyclist, she was described as a 'tall slight figure,…instinct with energy to the very tips of the tapering fingers – a pale face with bright blue eyes, and crowned with a wealth of light golden hair'.
A feminist and prohibitionist, Keller soon became acquainted with like-minded women. In September 1901 she was interviewed by Kate Sheppard on women's employment in medicine, and lectured on rational dress to the Christchurch branch of the New Zealand Women's Christian Temperance Union and the Canterbury Women's Institute. She promoted the 'comfort and health-giving effects of rational clothing', urging women to discard corsets, elastic garters, tight shoes and trailing skirts. A few months later she presented a paper on the ill effects of alcohol on the body.
In 1902 the Kellers left Christchurch to live in Sydney, returning to New Zealand around 1904 to practise medicine in the North Island, first at Huntly and later at Hikurangi. Florence travelled bush country by horse to tend patients in Maori communities, and through her friendship with Maui Pomare, also a Seventh-day Adventist medical practitioner, became a doctor to the King movement. In 1906 she returned to Walla Walla, Washington, to ensure that her only child, Frances, was born an American citizen. The Kellers subsequently moved to Auckland, where they established a medical practice at their home on Ponsonby Road. Around 1914 they shifted their surgery to Queen Street. Keller maintained her professional contacts in the United States with a tour of American children's hospitals in 1915 and a postgraduate course in surgery in Chicago in 1917.
In Auckland Florence Keller became involved in community work. She was on the Ponsonby School Committee from 1914 to 1915, and was a member of the Auckland Hospital and Charitable Aid Board from 1913 to 1919, first as an independent, then as a nominee of the Auckland Labour Representation Committee. She supported Eleanor Baker and Constance Frost, doctors at Auckland Hospital, when they faced discriminatory treatment by the board. Keller was an executive member of the Auckland branch of the National Schools Defence League, established to defend secularism in schools and oppose the activities of the Bible in State Schools League. She joined the Ponsonby branch of the Women's Christian Temperance Union of New Zealand, and in 1915 became the WCTU's superintendent of purity and moral education. In 1918 and 1919 she tutored a women's hygiene class for the Auckland Workers' Educational Association, giving 20 lectures on subjects such as venereal disease, invalid cookery, sanitary housing, school hygiene and first aid.
Keller and her family returned permanently to the United States in 1919. She worked in California at the College of Medical Evangelists, Linda Loma, San Bernardino, as professor of the department of obstetrics and gynaecology, and later became a surgeon at the White Memorial Hospital in Pasadena. She was elected a fellow of the American College of Surgeons and the International College of Surgeons. On 1 October 1931 Martin Keller was shot and killed by a patient at Glendale Sanitarium, California.
A few years before her retirement in her mid 90s, Florence Keller was acclaimed the oldest practising surgeon in the world. She died shortly before her 99th birthday in Los Angeles on 15 January 1974, survived by her daughter.