Ya shiga cikin Rundunar Sojoji na Biyu tare da matsayi na biyu a ranar 5 ga Oktoba, 1846. An canja shi zuwa sabuwar sabuwar ta 3rd US Dragoons a ranar 9 ga Afrilu, 1847. Ya yi yaƙi a cikin Yakin Mexico da Amurka kuma an ambaci shi don ƙarfin hali a lokacin yaƙe-yaƙe na
Churubusco da kuma Molino del Rey. A cikin aikin da aka yi a baya, Thorn da wasu jami'ai da dama sun ba da kansu don motsawa da kuma aiki a matsayin ma'aikata na bindigogi a karkashin jagorancin Kyaftin Simon H. Drum wanda ke da tasiri a kan yakin. An ci gaba da shi zuwa kyaftin a ranar 8 ga Satumba, 1847 don wannan aikin.