New Zealand's first Plunket nurse, Joanna MacKinnon was born in Balmeanach, on the island of Skye, Scotland, on 12 November 1878. She was the daughter of Jane Finlayson and her husband, John MacKinnon, a fisherman. It is not known when or why Joanna MacKinnon came to New Zealand but from 1902 she was employed as an attendant at Seacliff Lunatic Asylum (later Seacliff Mental Hospital) near Dunedin. The matron there, C. M. Beswick, recommended her to the superintendent, Frederic Truby King, and in 1905 he asked her to teach mothers in the village of Seacliff his methods of modifying cows' milk for infant feeding. She was not a registered nurse, nor was she experienced in infant care, but under King's intensive tuition she quickly and successfully established herself as the missionary of King's ideas concerning infant feeding and hygiene. He paid her himself and supervised her closely. In early 1906 he moved her to Dunedin to extend the influence of his scheme.
During the first months of her work Joanna MacKinnon responded to many calls from mothers and taught them how to 'humanise' cows' milk for their babies. Each day she would 'catch up the skirts of her grey uniform, tie her bonnet firmly under her chin and set out on her bicycle'. Regarded by King as a 'bright, winsome young woman' and a highly capable and receptive nurse, MacKinnon soon became a central figure in the developing infant welfare movement in Dunedin. She gave public lectures, answered letters to the newspapers, and supervised the setting up of a system of wholesale modification of cows' milk at the Taieri and Peninsula Milk Supply Company.
Joanna MacKinnon's personality and enthusiasm attracted the interest and support of middle-class women, some of them wives of doctors who were sceptical of King's ideas. These influential women became involved in establishing a society 'for Promoting the Health of Women and Children' in May 1907 to extend and financially support her work. MacKinnon drew the attention of King and the women of the society to the plight of neglected infants (mostly illegitimate) who were boarded out in licensed houses. After King had used his home and then his seaside cottage at Karitāne to board babies, the society set up the first secular institution for infant care in New Zealand, the Karitāne Home for Babies, at Andersons Bay in December 1907.
Although MacKinnon had some involvement with the Karitāne Home her main interest was in continuing home service to mothers and babies in the community. A formalised system based on her work was proposed in February 1908 by the governor, Lord Plunket, and his wife, Lady Victoria Plunket, patroness of the society. Under this scheme MacKinnon was appointed supervisor of the community work of nurses following their formal training in the care of infants at the Karitāne Home. In March 1908 it was decided to call the nurses Lady Plunket nurses, and they were given a medal at the completion of their course. The first medal was awarded to Joanna MacKinnon in April 1908. Although she was not a certificated Plunket nurse she was considered to be the pioneer of the movement.
While some of her colleagues felt that she got the public attention while others did the 'real work', she had helped to win over popular opinion and to lay the working foundation of the newly entitled Society for the Promotion of the Health of Women and Children (later the Plunket Society). She had also trained other leading practitioners of King's methods, including Alice Bowman, who became one of the first matrons of the Karitāne Home in Dunedin and later the first Plunket nurse in Timaru; and Evelyn McAdam, who was influential in establishing the first Presbyterian orphanage in New Zealand in July 1907.
In Dunedin Joanna MacKinnon boarded with John and Atalanta Murray in George Street, and she married their son James Dingwall Murray, a fireman, on 22 July 1908. The couple were to have two sons. After her marriage Joanna Murray continued to go on lecture tours to promote the society and assisted in the establishment of new Plunket nursing districts, but she resigned from her position in the Dunedin area. She remained in service to the Plunket Society until her formal resignation in 1930, but her main involvement spanned a brief period of four years.
Throughout her life Joanna MacKinnon remained committed to the ideals of the society. Her motto was 'Once a Plunket Nurse always a Plunket Nurse,' and this was exemplified when, at the age of 78, she responded to an appeal from Dunedin Hospital to care for a sick baby. Following the death of her husband, James, on 2 June 1960, Joanna went to England to stay with her son and visit her homeland in Skye. She returned to New Zealand after her son's death and died at the Central Mission Home, Dunedin, on 26 August 1966.