Hilda Margaret Northcroft was born in Hamilton on 22 April 1882, the daughter of Henry William Northcroft, a resident magistrate, and his wife, Margaret Henderson. Her father was English and her mother Canadian. Hilda attended Auckland Grammar School, gaining a Junior Scholarship in 1900. After one year at Auckland University College she transferred to the Medical College for Women in the University of Edinburgh. She graduated MB, ChB in 1908 and trained in obstetrics at the Rotunda Hospital, Dublin, gaining her LM in 1911. During the First World War she practised medicine in England and returned to New Zealand in December 1918 as medical officer on the troopship Ayrshire.
Hilda Northcroft registered as a doctor in Auckland on 27 May 1919. She specialised in diseases of women and children and practised obstetrics: she was reputedly the first in Auckland to use 'twilight sleep' for painless childbirth. As a doctor she was popular among the women of Auckland, but would make her mark, not so much in medicine as through participation in many committees, where she promoted women's issues.
In 1919 she was on the committee of the Auckland branch of the Royal New Zealand Society for the Health of Women and Children (the Plunket Society), and from 1920 to 1932 she was a member of its advisory board. A member of the Auckland Hospital Board from 1938 to 1947, she topped the polls in 1941 and 1944 and played a vigorous part in developing the new Green Lane Hospital. During the Second World War she was involved with the Auckland district committee of the Women's War Service Auxiliary.
Northcroft became secretary of the Auckland branch of the New Zealand Obstetrical Society after its foundation in 1927. She was active in the New Zealand Medical Women's Association (NZMWA) from its beginnings in the early 1920s, and in 1924 was elected vice president for Auckland. In the 1930s she lobbied medical women to join her at British Medical Association divisional meetings to discuss the introduction of social security. The NZMWA Auckland branch elected her a life member in 1947 in recognition of her work for medical women.
When the International Federation of University Women (IFUW) established its Auckland branch in 1920, Northcroft joined the local executive, becoming president from 1921 to 1925. On her retirement it was recorded that 'without the energy and interest of Dr Northcroft, the Auckland Branch of the Federation of University Women would probably have lapsed'. From 1925 to 1929 she was dominion president of the IFUW and in 1932, during a visit to Britain, she attended a conference of the organisation in Edinburgh.
Northcroft became involved with the National Council of Women of New Zealand (NCW) in 1921 when the IFUW Auckland branch decided to affiliate and chose her as its first delegate. The following year she became national secretary of the NCW, and vice president in 1925. She was also NCW Auckland branch president in 1922–23 and from 1924 to 1927. She represented New Zealand on the International Council of Women from 1932 to 1942. Through the NCW she lobbied for women to be made justices of the peace, and was herself one of the first to be appointed, in 1926.
Her concern for women's interests took Northcroft into national politics. In the 1920s she was active in the Reform Party's women's auxiliaries and in 1936 she helped form the New Zealand National Party at the dominion conference of the coalition survivors, Reform and United. She was first chairwoman of the Auckland women's section of the party and toured the divisional area, holding meetings and establishing other sections: by 1938 there was an efficient women's section in every electorate in the division. She became one of the party's first dominion councillors in 1937, representing Auckland. In 1938 she persuaded council to give women voting rights, and was largely responsible for the decision that each division should try to have at least one woman dominion councillor. She was deputy chairwoman of the Auckland division of the National Party between 1941 and 1945 and women's vice president (North Island) in 1943–44.
Northcroft had a forceful personality, was extremely energetic, and gave herself unquestioningly to the service of the women of New Zealand. She welcomed people into her home, where she hosted meetings and gatherings. Her leisure pursuits included gardening. Hilda Northcroft never married. She died at Green Lane Hospital, Auckland, on 14 June 1951.