 He then served as preceptor of the children of Prince Colonna, and as professor of philosophy and theology at the Pontifical Urban Athenaeum of "Propaganda Fide".After being raised to the rank of Privy Chamberlain of His Holiness, he was made auditor of the nunciature to Spain and Domestic Prelate of His Holiness (1857).Simeoni later became "adiutor ab actis" of the Sacred Congregation for the Propagation of the Faith, and served as the Congregation's secretary from 1868 to 1875.