Kammala jumla mai zuwa: Smith ya yi aiki a matsayin colonel na masu sa kai a cikin Seminole Wars daga 1836 zuwa 1838, kafin ya shiga cikin Mexican-American War. Ya umurci Brigade na 2 a cikin Worth's Division a yakin Monterrey. An ba shi izinin zama brigadier janar a watan Satumbar 1846 kuma ya shiga rundunar Winfield Scott a matsayin kwamandan Brigade na 1, 2nd Division. Ya jagoranci tawagarsa a yakin da aka yi a
Veracruz, Cerro Gordo da kuma Contreras. An ba shi babban janar don ayyukan a Contreras kuma ya yi yaƙi a yaƙe-yaƙe don Mexico City. Ya yi aiki a kan kwamiti na armistice sannan kuma a matsayin gwamnan soja na Mexico City. Ya kasance memba na asali na Aztec Club na 1847.