Hermione Ruth Herrick was the youngest of eight children of Emily Martha Duncan and her husband, Jasper Lucas Herrick, a well-to-do sheepfarmer and an early settler in the Hawke's Bay area. She was born at Forest Gate, Ruataniwha, on 19 January 1889. Her father died when she was nearly two years old. With her mother occupied in running the family farm, Ruth had considerable freedom and developed her knowledge of native trees and plants, and her love of animals. She was described by her family as headstrong and independent. Later, she lived with her elder sister, Hetty, in London and attended Queen's College, where she befriended Kathleen Beauchamp (Katherine Mansfield). She then travelled extensively in Europe, continuing her piano studies in Vienna and Dresden. During the First World War she worked as secretary to the head of the Nursing Division at Walton-on-Thames, England. Afterwards, she returned to New Zealand to live with her mother in Napier.
In 1929 Ruth Herrick's friend Enid Bell, the deputy chief commissioner for the Girl Guides Association in New Zealand, persuaded her to become provincial commissioner for girl guides in Hawke's Bay. She left for England in 1931 to learn more about the Girl Guide movement. There she had close contact with Olave Baden-Powell, the chief guide, and her husband, Robert, who, with his sister Agnes had founded the movement. She attended the seventh conference of the World Association of Girl Guides and Girl Scouts in Poland, and also went to a number of girl guide centres and smaller conferences in England. This experience 'enlarged her outlook' and showed her 'boundless possibilities' in establishing better understanding among young people of different races. In 1932, after being appointed deputy chief commissioner for girl guides in New Zealand in her absence, she returned home, and in 1934, on the recommendation of Imperial Headquarters, London, she was appointed chief commissioner for New Zealand, a position she was to hold for 27 years.
In 1942 the Royal New Zealand Navy began to recruit women and Ruth Herrick was invited to become director of the Women's Royal New Zealand Naval Service – initially known as the Women's Royal Naval Service (New Zealand). Given leave of absence from the Girl Guide Association, she took up this appointment at the Navy Office in Stout Street, Wellington, on 2 June 1942. Herrick began with the rank of chief officer, the only director of the WRNZNS to achieve this status. Her task was to form an administration and a policy for the WRNZNS based on the regulations of the WRNS in Britain, but adapted to New Zealand circumstances. With the assistance of her deputy director, Helen Fenwick, she also had responsibility for recruiting and training. This was no small task for someone from a civilian background, working with limited resources. Her high standards of selection were shown by her rejection of over one-third of the applicants up to the beginning of 1943. Fenwick remembered her as a 'born leader, a strict disciplinarian, but also very fair'. In December 1946, with the disbanding of the WRNZNS, she was discharged. A citation from the New Zealand Naval Board praised 'her loyalty, determination and hard work in the face of many difficulties and frustrations'. That year she was made an OBE in recognition of her military service.
She returned to her position as chief commissioner of the Girl Guides Association. While she was in charge, camping and other outdoor activities burgeoned and the association enjoyed a high public profile. The Silver Fish, the highest award in guiding, was awarded to her in 1949, in 1962 she was made a CBE for her services to youth, and in 1966 the National Council appointed her a national vice president for life. In 1961 the new headquarters in Christchurch was named Herrick House in her honour.
She is remembered by many with admiration and respect, although some stood in awe of her and found her difficult to relate to; Lady Fergusson, wife of the governor general, criticised the Girl Guide movement in 1963 for not doing enough to help or include Maori. Herrick's belief in discipline and strong leadership was to some extent carried over from her naval experience into guiding. Her sense of humour and willingness to tackle menial jobs were appreciated by those who worked with her. After retiring as chief commissioner in 1965, she returned to Wellington to live. She never married, and died at her home in Wellington on 21 January 1983.