She was at the height of her popularity from the late 1950s to the 1980s, when many of her novels were translated into other languages. "The Moon-Spinners", one of her most popular novels, was also made into a Disney movie. Stewart was one of the most prominent writers of the romantic suspense subgenre, blending romance novels and mystery. Critically, her works are considered superior to those of other acclaimed romantic suspense novelists, such as Victoria Holt and Phyllis Whitney. She seamlessly combined the two genres, maintaining a full mystery while focusing on the courtship between two people, so that the process of solving the mystery "helps to illuminate" the hero's personality—thereby helping the heroine to fall in love with him.