Both the date and place of Johnston's birth are unknown; it has been suggested, and is generally accepted, that she was born in northwestern France, near the town of Rennes. Her parents, both French Huguenots, were Francis (possibly Cézar) and Suzanna de Beaulieu, and the family immigrated to London in either 1685 or 1687. In 1694 Henrietta married Robert (possibly William) Dering, fifth son of Sir Edward Dering, 2nd Baronet, and his wife Mary; she and her husband then moved to Ireland. It was during this time that Johnston began to draw pastels. Her earliest portraits depicted a number of powerful people to whom she was related by marriage; among these were John Perceval, later to become Earl of Egmont, and one of the Earls of Barrymore. Her earliest surviving pastel dates to 1704.