Pierre-François-Marie-Auguste Dejean was the son of Count Jean-François-Aimé Dejean (1749-1824), a close aid of Napoleon and Ministre de l'Administration de la guerre, and Alexandrine-Marie-Elisabeth Le Boucher d'Ailly.
Born in Amiens on 10 August 1780, the young Dejean was a precocious child, winning a silver medal for his work on botany at the age of nine, as depicted on the table in the present portrait, and later wrote books on the subject. He went on to have a distinguished career as a cavalry officer, becoming aide de campe to the Emperor in 1813. He was awarded the Légion d'honneur in 1844, dying the following year. He was buried next to his father in the cemetary of Pére Lachaise, Paris.