From 1942 to 1944 Appel studied at the Rijksakademie in Amsterdam, where he met Corneille. Together with Constant and Corneille he founded the 'Groupe Experimental Hollandais' in 1948. In the same year Dotremont, Noiret, Jorn, Constant, Corneille and Appel founded the artist movement COBRA. In 1958 he painted the murals for the Dutch pavilion at the World Exhibition in Brussels and the UNESCO building in Paris. During the exhibition 'Vita lita nell' arte' Appel decorated one room with a cloth collage in the Centro Internazionale delle Arti e del Costume in the Palazzo Grassi in Venice. In 1964 Appel bought Castle Molesmes near Auxerre, where he produced large polychrome reliefs and freestanding sculptures of wood and polyester. In 1984 Appel was awarded the 'Grand Prix du Salon' at the XXIX. Salon de Montrouge in Paris. In 1991 he worked with Allen Ginsberg and Gregory Corso on the Poetry-Painting Series. In 1994/95 Appel concentrated on stage design at the Dutch opera.