His portraits often adopted the pastoral style popular among the Dutch elite. In 'Portrait of a Woman as a Shepherdess' and 'Portrait of a Man as a Shepherd', the sitters appear in theatrical costumes. These works are not literal depictions of rural life. Instead, they represent a fashionable fantasy of simplicity. De Heer used fine lines to define the texture of fabric and the softness of hair. His compositions are balanced and clear. He avoided the heavy shadows common in the school of Rembrandt, preferring an even distribution of light.

Modern collectors value De Heer for the graphic quality of his work. His drawings and pen-paintings have a monochromatic clarity that suits contemporary spaces. The specific focus on individual plants or stylised figures makes his art easy to pair with different decor styles. His botanical studies provide a historical look at early scientific illustration. These prints offer a sophisticated alternative to more traditional Dutch paintings. His work is defined by technical mastery of line and form.