Marianne Hart was one of two children of John Hart (merchant) of Pt Adelaide and Semaphore, and Emily Lavinia (nee Finch). Her father, who had taken over the family business and inherited the family home, Glanville Hall, at Semaphore, on the death of his father in 1873, represented Pt Adelaide in the House of Assembly 1880-1. He died of consumption at the age of 33 in August 1881, when Hart was an infant.
The family continued to live at Glanville Hall until 1912 when they sold the house. They moved to Northgate St, Unley Park and later (c.1940) to Brougham Place, North Adelaide. Hart followed her mother as Secretary of the Committee of Management of St Margaret's Hospital, Semaphore (originally the Cottage Convalescent Home, founded by her father's sister, Margaret Hart, c.1872).
Hart had artistic talent, and painted watercolours. She undertook the task of copying out in her neat handwriting the 1865-1872 diaries of her grandfather, "Captain" John Hart (1809-1873), a well-known SA identity in his day (see Australian Dictionary of Biography 1851-90). In 1936 Hart published A Sketch: 1936, the Year of South Australia's Centenary, about her grandfather.
