Under the Empire, Lefebvre-Desnouettes fought with distinction at the Battle of Elchingen in 1805. Later that year, he became colonel after the Battle of Austerlitz. He served also in the Prussian campaigns of 1806-1807. He was promoted to general of brigade in September 1806 and general of division in November 1807. He was created a count of the Empire in March 1808.