Johnstone was born in Edinburgh in 1892 and grew up in Napier Road, near the Gothic Mansion, Rockville.

She took the Life Class with Ernest Stephen Lumsden where she revealed her talents at informal portraiture, a genre for which she became well known.

As a consequence of her husband's appointment in 1933 as head of Gray's School of Art in Aberdeen (now at Robert Gordon's University) and of the marriage bar in place at the time, she gave up her career and her students.

She kept her links with Edinburgh by continuing to exhibit her portraits and landscapes at the Royal Scottish Academy, to which she was elected an Associate (ARSA) in 1962.

Johnstone painted landscapes and portraits, particularly of children, and her style was free and relaxed, whether using oil, watercolour, pencil or chalk.