Lejeune's path to becoming an engineer was most unusual. He was not a graduate of either the excellent engineer school at Mezieres nor was he a member of the Service des Ponts et Chausses (the Bridges and Roads Service). He was appointed as an adjoint du genie when he was a supernumerary on the staff of General Philippe Jacob during the wars of the Revolution and had found a ford over a river which was witnessed by the Representatives of the People with that command and was made a sous-lieutenant of engineers as a reward for his efforts. Lejeune took his new responsibilities seriously and studied his new profession in his off time to become qualified.