Valadier began his career in 1781 as an architect of the papal palace under Pope Pius VI .. Also during the occupation of Rome by the French during the Napoleonic era in the years 1809 to 1814 and Pope Pius VII he retained this position.
1790, Pope Pius VI. acquired the Palazzo Santo Bono at the Piazza Navona for his nephew ' Luigi Braschi Onesti - ' and commissioned, among other things Valadier with plans for the renovation of the palace. The Pope rejected these plans, however, and had rebuilt the palace by Cosimo Morelli in a style that was based on the Renaissance. This fit well to the cultural and political intentions of Pius VI. , But also simultaneously provides an explanation for that neoclassicism was later than in the rest of Europe unfold in Rome. Had reached only when Valadier the height of his creative powers, Rome followed the developments in other European centers.
Built as an architect Valadier including the Teatro Valle ( 1819) and the facade of San Rocco (1831). As an archaeologist, he worked from 1819 to 1822 with the restoration of the Colosseum and the Arch of Titus, and from 1829 to 1835 with the restoration of the Temple of Portunus (Temple of Fortuna Virilis ).
However, most legacy he had left as a town planner and outstanding musician. Throughout its steeply rising career as a city planner, he had early leave behind this talent. In 1805 he established the new course of the Via Flaminia. 1811 followed the general plan for the Via dei Fori Imperiali, but was then first brought by the Nazis to carry out. Next, he led the transformation of the Piazza San Giovanni in Laterano and the design of the Villa Torlonia in Rome. However, the masterpiece Valadier was undoubtedly the redesign of the Piazza del Popolo. He had already presented a first draft in 1793, of which, however, nothing eventually applied in the final version of the rebuilt 1811-1822 square.