Daughter of James Douglas and Mary Maxwell, daughter of Patrick Maxwell, 1st Bart of Springkell. She was married twice, firstly to William Richard Middlemore (d. 1772) with whom she had several children. Around 1777 she married Dr John Storer (1747-1837), son of the Rev. John Storer of Fossoway, Kinross.
Boswell met Mary Middlemore, as she then was, and her husband at Sir John Douglas' Kelhead estate on October 6, 1762. He described her as "a pretty, agreeable young creature".