Born in Milan, it had been intended that he follow his father's career in medicine but instead entered the private academy of the painter Carlo Maria Giudici (1723–1804) where he received instruction in drawing, copying mainly from sculpture and prints.

From there, he then joined the class of the fresco painter Antonio de' Giorgi, which was held at the Ambrosiana picture gallery in Milan.

Appiani's interest in aesthetic issues was stimulated by the classical poet Giuseppe Parini, whom he drew in two fine pencil portraits.

In 1776 he entered the Brera Academy of Fine Arts to follow the painting courses of Giulio Traballesi, receiving a mastery of the fresco technique.

During his period as court painter he rendered portraits of Napoleon and the chief personages of his regime, among the most graceful of which are his oil paintings Venus and Love, and Rinaldo in the garden of Armida.