Born in Edinburgh, Johnstone was supported by her father, a renowned landscape artist, in her pursuit of a career in the arts. At the age of sixteen Johnstone enrolled as a student at the Edinburgh College of Art, where she excelled in portraiture, and continued to teach at the College after graduation. The artist regularly travelled to Galloway, where she would take great pleasure in painting en plein air. The artist enjoyed a long career as an artistic educator alongside her professional pursuit of painting. In in 1962 the artist was elected an Associate of the Royal Scottish Academy.