British social worker. Harriet Mason belonged to a family of prominent Anglican activists of the late Victorian era. Her younger sister, Frances Agnes, was the founder of the Community of the Holy Family; her eldest brother, William Henry, was a member of the high-church Woodard Corporation; and another brother Arthur James, was a theologian at Cambridge and chaplain to the Archbishop of Canterbury. Born in Marylebone, Middlesex, Harriet Mason resided from 1869 with her family at Morton Hall, Nottinghamshire, where her father, George Mason, was justice of the peace and deputy lieutenant.

Government policy at this time was to take poor-law children (orphans, deserted children, and the children of convicts) out of the workhouse and place them with paid foster families. In 1885, after many years' experience working as voluntary supervisor of the poor-law boarding-out committees in Nottinghamshire, Harriet Mason was commissioned by the Local Government Board to make an official report on the living conditions of boarded-out children who had been sent to homes "outside the Union". (The Poor Law Amendment Act of 1834 divided the parishes of England and Wales into new administrative units called Poor Law Unions, each run by a locally elected Board of Guardians, which looked after the workhouse and the children boarded within the union. Those children boarded outside the union, however, were the responsibility of the Local Government Board.) The report she submitted, which emphasised the need for systematic government inspection, led to her appointment as the first official Woman Inspector of boarded-out children.

In her annual reports, she wrote with great candour of her visits to foster homes across England and Wales, about 300 a year, and expressed her forthright views on the standards of childcare then in practice. She repeatedly criticized local poor-law guardians for failing to carry out regular inspections within their unions, and demanded that visits include physical examinations of the children for signs of abuse and neglect. Over the years she showed remarkable commitment to her work and stayed in contact with many of the children she had met, helping them to find work and housing long after they had grown up. Despite her appeals for more women inspectors, whom she believed were better qualified to carry out physical examinations, she worked alone until 1898 when exhaustion finally caused her health to fail. The board was forced to hire a deputy woman inspector during her illness, whose assistance, once she returned to work, gave her more time to write pamphlets and articles and accept speaking engagements. Because of her special knowledge of the poor law as it related to children, she was often asked to give talks about her work to interested groups, and in 1909, a year before her retirement, she was summoned as a witness to give evidence to the Royal Commission on the Poor Laws.

She retired to South Africa, where her brother, Canon Edward Mason, was principal of St Bede's Theological College at Umtata, Transkei, and eventually settled in Cape Town. They travelled widely together, visiting the Cape peninsula, Pondoland, Lesotho, Natal, Transvaal, Rhodesia, Uganda, and elsewhere. During these leisurely journeys she collected seeds and specimens, which she sent to Kew, and made detailed watercolour studies of the flora they encountered. She joined the South African Botanical Society and published "Some flowers of eastern and central Africa" in the Journal of the Royal Horticultural Society in 1913, the same year she was elected as a fellow of the Royal Geographical Society. She presented her botanical paintings and sketches to Kew, where she was a regular visitor whenever she returned to her home in London. She kept up her work with various charitable societies in London and, as a member of the Psychical Research Society, published articles about her experiments in thought-reading. Plant species named after her include Indigofera masoniae N.E. Br., Watsonia masoniae L. Bolus and the popular European garden plant Crocosmia masonorum (L. Bolus) N.E. Br.