Mary Manson Bain was born in Dunedin on 31 March 1887, the first of thirteen children of Alexander Manson Bain, a cabinetmaker, and his wife, Hanna Kiely. Her parents, he a Scottish Presbyterian and she an Irish Catholic, had arrived in New Zealand in 1886. Mary was brought up in the Anglican church, and attended primary school in Dunedin. Her father was a dedicated trade unionist and, following his example, Mary developed a strong sense of social concern. An outgoing and enterprising person, when about 18 years old she began to supplement her limited formal education by participating in public-speaking competitions.
After her marriage on 12 April 1911 in Christchurch to Andrew James Dreaver, a travelling salesman and former Otago–Southland featherweight boxing champion, Mary moved to Auckland. The couple had six children between 1912 and 1925. During this time Mary attended classes at Seddon Memorial Technical College and extension courses at Auckland University College, and obtained a licentiate of the London College of Music in piano. Her piano teaching was an important source of income for the family during the depression, while her performing skills were to be an asset in her public life: 'I used to sing and recite,' she said in 1941, 'and this gave me confidence on a public platform as well as enabling me to speak clearly and fluently'.
Dreaver belonged to a large number of organisations, and commonly held positions of responsibility in them. She joined the Spiritualist Church of New Zealand and was ordained a minister around 1938. At three different periods she hosted a radio programme and for a time during the 1940s she wrote an astrology column for the New Zealand Woman's Weekly. As a result she achieved a high level of public attention, and approval, in her local community – an ideal basis for a political career. Her clearly stated reasons for wishing to be in politics were quite conventional: the welfare of women and children, and health care.
In pursuit of these goals, Dreaver in 1922 joined the Women's International and Political League, which in 1925 became the Auckland women's branch of the New Zealand Labour Party. In 1923 the League elected her its delegate to the National Council of Women of New Zealand. Despite the opinion of some in the party hierarchy that it should concentrate on fund-raising and social activity, the women's branch was a politically assertive body and Dreaver was president of it in 1926–27 and from 1936 to 1938. As the branch's nominee, she spoke at public meetings on behalf of the Labour Party, attended annual party conferences and sat on various committees (such as one in 1926 seeking improved relief-work wages for the unemployed). At the 1936 conference she moved a remit seeking equal representation for women on the boards administering mental hospitals, and another calling for the appointment of women police officers.
The branch also regularly nominated her for inclusion on Labour electoral tickets, and she had many successes in municipal contests. In the male-dominated spheres into which she entered she was respected for her formidable grasp of procedure. She was a member of the Auckland Hospital Board (1933–44 and 1950–56); the Auckland City Council (1938–44 and 1953–61, its second woman member after Ellen Melville); the Auckland Transport Board (1939–44); the Auckland Electric Power Board (1944–47); and the Auckland Metropolitan Drainage Board (1956–57).
In general elections she was less successful, but still significant as a trail-blazer. She sought (unsuccessfully) Labour nomination for the Grey Lynn electorate in 1931 and vainly contested the Remuera seat in 1938. In 1941, however, she won the Waitemata seat in a by-election, becoming the third woman to enter the New Zealand Parliament. Her particular triumph there was to introduce the Women Jurors Bill, which became law in 1942. She was defeated in the 1943 election. From 1942 to 1945 she was also an organiser of the Women's Land Service. For this work she was appointed an MBE in 1946. The same year she was appointed to the Legislative Council, and remained there until it was abolished in 1950.
As a pioneer of women's participation in public life, Mary Dreaver was exceptional, yet as a person she was, in many respects, quite conventional. A robust, solidly built woman, she was rarely seen not wearing gloves, a fox fur and jewellery or without a still life of flowers or fruit adorning her bodice. For leisure she swam regularly and painted watercolour landscapes, which she exhibited under the name of May Bain.
She died in Auckland on 19 July 1961. Her husband, Andrew, died three months later. They were survived by three daughters and two sons. Another son, Pilot Officer Bruce Dreaver, had been killed while flying in Britain in 1943.