In 1812, during the French invasion of Russia, he was made général de brigade and chief of staff to Davout. Frostbitten on the face, Lejeune left his post during the retreat from Russia and was arrested on the orders of Napoleon. Freed in March 1813, Lejeune was then sent to the Illyrian provinces, before rejoining the army under the orders of marshal Oudinot, becoming his chief of staff. During the Saxony campaign, Lejeune was present at the Battle of Lutzen (1813), the crossing of the River Spree and at Bautzen. He was made an officer of the Légion d'honneur and a commander of the Order of Maximilian of Bavaria. At the battle of Hoyersverda, when Bülow's corps wiped out the 12th corps formed up in square on the plain, Lejeune (at risk of being kidnapped) ventured into the enemy lines with one battalion, general Wolf's cavalry and six 12 pounder guns. He thus broke the whole of the Prussian artillery and saved marshal Oudinot and his army. Wounded several times and lastly at Hanau, he was authorised to leave the army in November 1813 after more than 20 years' service. After his departure from the army, he devoted himself to painting.