Lejeune took part in many of the Napoleonic campaigns, and his vigorous battle pictures, executed mainly from sketches and studies made in the field, enjoyed a great vogue. The campaign of 1806 took him to Munich, where he visited the workshop of Alois Senefelder (Senefelder, Alois), inventor of lithography. Fascinated, Lejeune drew on the stone his famous Cosaque and immediately pulled 100 proofs of it, one of which he later submitted to Napoleon.

Lejeune also painted many landscapes and portraits. In 1837 he became director of the École des Beaux-Arts and curator of the museum at Toulouse.