One of the major works in his oeuvre is the Theodoli Chapel in the Basilica of Santa Maria del Popolo which is a gem of Roman Mannerism. Mazzoni was commissioned by Traiano Alicorni in 1555 to decorate his chapel in the basilica. The work was restarted by a new patron, Girolamo Theodoli, the Bishop of Cádiz after 1569 and finished around 1575. Mazzoni created a harmonious and iconographically coherent ensemble of delicate stuccos, frescos and paintings with the classicising statue of Saint Catherine in the focus of the space.
^Patrizia Tosini: La cappella Alicorni Theodoli e la decorazione di Giulio Mazzoni da Piacenza, in I. Miarelli Mariani, M. Richiello (a cura di), Santa Maria del Popolo. Storia e restauri, 2 voll., Poligrafico dello Stato, Roma 2009, II, pp. 489-507