Mary McQueen was born in Ballarat, Victoria, Australia, on 13 July 1860, the daughter of John McQueen, a farmer, and his wife, Catherine McIntosh. Little is known about her early life. She trained at the Presbyterian Deaconess Training Institute in Melbourne in 1900 and 1901 and was made a deaconess by the Presbyterian Church of Victoria on 7 May 1902. Later that year, when aged 42, she accepted an appointment in New Zealand as full-time parish deaconess at Knox Church, Dunedin, to assist in the charitable work of the congregation.
Sister Mary, as she was known, worked in conjunction with the Ladies' Association at Knox Church, caring for the poor, aged and sick of the parish, and visiting to offer both spiritual support and material assistance. Her work brought her in contact with many outside the membership of any church. The association raised funds for coal, provisions, blankets and clothes for her to distribute. Her salary of about £70 (raised to £80 at the end of her first year's work) was provided by the deacons' court from the proceeds of organ recitals and individual contributions.
Remembered as 'quiet and unobtrusive to a degree', Sister Mary, with Sister Evelyn McAdam of First Church, soon began to urge a more vigorous Presbyterian response to the social needs of Dunedin. In 1906 they were jointly appointed to visit Dunedin hospital patients and they helped to found the Presbyterian Social Service Association in Otago. By early 1907 they were caring for a number of children in their George Street accommodation, and because of urgent need and a degree of denominational rivalry the association set up the Presbyterian Orphanage and Children's Home at 41 Clyde Street.
In July that year Sister Mary resigned from Knox Church to be the first matron of the orphanage. Thereafter she worked with the Reverend E. A. Axelsen, first superintendent of the Otago PSSA, to develop homes that provided a stable happy environment for orphaned or neglected children, a number of whom were referred each year by the Juvenile Court.
Over the next 15 years Sister Mary stamped her personality on the orphanage and home, and on the rapidly developing child-care programme of the PSSA, becoming matron of the larger Grant's Braes home in 1908 and Glendining home in 1912. During the influenza epidemic of 1918–19 she established a convalescent home for children and a temporary children's home to meet the extra need.
While public opinion was urging the churches to accept the onus of controlling neglected children, Sister Mary was more concerned with their nurture and encouragement. She was an attractive, warm and generous woman, whose strong personality and sensible attitudes enabled her, in the face of contemporary practice, to do away with harsh discipline and corporal punishment. Cheerful, with a sense of humour, she could see things from a child's viewpoint.
In May 1922 Sister Mary announced her intention to return to Australia in the following month. During a long and active retirement in Brisbane she continued to follow the affairs of the PSSA with close interest and with regular financial support, and to maintain an active local involvement in hospital visiting, Sunday school teaching and the Presbyterian Women's Missionary Union. She corresponded with former residents of the Dunedin children's homes right up to her death in Brisbane on 30 May 1945.
Mary McQueen had come to Dunedin as one of the first group of deaconesses trained in Australia, where the Presbyterian church had already begun to make the transition from parish-based charitable aid to institutionalised social services. The Otago PSSA, which Mary McQueen had helped to establish, became the model for a national network of such associations, later known as Presbyterian Support Services.