Mabel was born into a family of school teachers, her mother, father and aunt were teachers and Mabel and her sisters, Maude and Gertrude became teachers. Mabel was baptised on 11 October 1843 in Holborn, Middlesex, her father, Charles Emilius Johnstone, was 37, her mother, Eliza Johnstone, nee Hitchcock, was 33 and her sister Maude was a baby. Her sister Gertrude was born 2 years later and her brothers Charles and Edward were born in 1849 and 1853 respectively.
By 1851, Mabel and her sister Maude were living with their aunt, Anne Bigg, a governess, in Clifton, Bristol while their cousin Alfred was living with her parents in Beaconsfield as a pupil at their school. Her father died in 1860 in Woodspeen, Newbury, Berkshire and the family moved to Worthing in Sussex.
Her mother, Eliza, was an entrepreneurial widow and set up a school for young ladies at Marlborough House, Marine Parade, Worthing. It was a family affair with cousins joining them at the school and the three daughters working as governesses or teachers at Marlborough, later known as Argyll House, on Marine Parade where their pupils boarded.
Her sister, Maude, married in 1877 and her mother died in 1892. In 1901 Maude was living on Marine Parade with a servant before she moved to join her sisters, Maude (widowed) and Gertrude at Maude's home, Woodspeen on Wordsworth Road, Worthing.
She died on 24 January 1925 at St. Boltolph's Nursing Home at the age of 81, and was buried in Heene Cemetery, alongside her sister Maude and brother-in-law, John Forbes Hardy in a grand plot purchased as a family grave by Maude.
She left her estate including her home, Woodspeen, 7 Wordsworth Road, Worthing, to her sister Gertrude, her sister-in-law, Jessie Isabel Johnstone and her nephew, Rev Rupert Johnstone, a clerk in holy orders.