In pose and coloring, many of Johnston's portraits strongly resemble those of Sir Godfrey Kneller, which at the time were greatly in fashion in the United Kingdom and the colonies. Her pastels from Ireland are drawn in deep earth tones, while those from her time in South Carolina are generally lighter and smaller, due likely to the precious nature of her materials, which had to be imported. The Irish works, which show the most attention to detail of all her works, depict sitters at three-quarter length, as do the earliest of her Carolina pastels. Johnston's American female subjects are usually shown wearing chemises, while the male subjects are drawn mostly in street clothes; some of the latter are depicted wearing armor. Each subject is shown sitting erect, with the head frequently turned at a slight angle from the body and towards the viewer. The faces are typically dominated by large oval eyes. Works dating to after her second husband's death are less finished; details of clothing are less well-defined and colors are less saturated, suggesting either that the artist was running out of materials or that she was working at greater speed to complete commissions.