His artistic approach favoured the heroic landscape style, often drawing from the balanced compositions of French and Italian masters. In works such as Flight into Egypt and Christ and the Woman of Samaria, Cossiau used clear lines and structured space to create a sense of calm. He was also a skilled etcher, as seen in his Liggende koe, where he captured natural subjects with a directness that avoided unnecessary ornamentation. His copies after Louis de Boulogne and Michel Corneille demonstrate a disciplined ability to translate complex oil paintings into the medium of print.

Modern collectors value these prints for their formal clarity and historical accuracy. The work of Jan Joost van Cossiau offers a window into the Baroque period without the visual clutter often associated with that era. His etchings, including Girl with Flute and his religious scenes, provide a sense of order and quietude. These pieces suit contemporary interiors that require art with a strong sense of structure and a connection to the classical tradition of European printmaking.