At about age seven, painter's son Guillaume Courtois left France for Italy with his teenage brother. They followed the military campaigns that his brother drew, then studied in Bologna, Florence, and Siena, arriving in Rome by 1638.
Courtois trained himself by copying Giovanni Lanfranco's and Andrea Sacchi's works and by drawing from nude models. He first studied under Pietro da Cortona, his most important teacher, while painting a fresco in an elegant Roman palazzo in 1656. In the following year he completed his first major public project, two frescoes for a Roman church. They display a strong sense of drama and a more naturalistic approach to the figure than did his later work, which followed the popular classicizing manner of Roman late Baroque painting. Courtois made many red chalk studies that demonstrate his great care in preparing the figures for his paintings. He also made composition drawings in pen, ink, and wash.