Kammala wannan jumla: A 1846, a farkon yakin Mexico da Amurka, an sake nada Roberts a matsayin na farko, Mounted Rifles, a cikin Rundunar Sojoji. An daukaka shi zuwa kyaftin a 1847, kuma ya ga aikin a Veracruz, Cerro Gordo, Contreras,
Churubusco, da kuma kama birnin Mexico, Matamoros da Galajara wuce. A Churubusco, an ba shi babban jagora don jagorancin ƙungiyar masu kai hare-hare. Ya sami ƙarin takardar shaidar, ga Lieutenant Colonel a 1847 don ƙarfin hali a lokacin yakin. Bayan karshen tashin hankali, ya yi aiki a kan iyaka da Washington, DC.