Grandjean was awarded the Commander's Cross of the Légion d'Honneur in 1804 and elevated in rank to general of division in 1805. During the War of the Fourth Coalition he led a division at Stralsund and Kolberg. Transferring to Spain he fought at the First and Second Sieges of Zaragoza in 1808–09. Later that year he led a division at the Battle of Wagram. Grandjean and his division participated in the 1812 French invasion of Russia after which they were besieged and captured at Danzig in 1813. He rallied to Napoleon during the Hundred Days and was placed on the inactive list. In 1821 he was elected to the Chamber of Deputies. His surname is one of the names inscribed under the Arc de Triomphe, on Column 16.