Initially he worked in the laboratory of Louis Nicolas Vauquelin, and in 1828 was named professor of chemistry and physics at the École Royale Vétérinaire d’Alfort (Royal School of Veterinary) in Maisons-Alfort.
In 1825 Lassaigne partnered with François Leuret to publish "Recherches physiques et chimiques pour servir à l’historie de la digestion" (Physical and chemical research for understanding digestion).
Lassaigne discovered new alkaloids and made major investigations related to toxicology of phosphorus and hydrocyanic acid.
He also discovered new dyes, and in 1831 won an award by "Société d'Encouragement de l'Industrie" (Society for Encouragement of Industry) for his work on the process of enamel elaboration for pottery.
In 1843, Lassaigne presented a procedure for detecting the presence of nitrogen in organic compounds by heating them with molten potassium.