Kammala jumla mai zuwa: Longstreet ya yi aiki tare da bambanci a cikin Mexican-American War tare da 8th US Infantry. Ya yi yaƙi a ƙarƙashin Zachary Taylor a matsayin laftanar a watan Mayu 1846 a cikin yaƙe-yaƙe na
Palo Alto da kuma Resaca de la Palma. Ya ba da labarin duka waɗannan yaƙe-yaƙe a cikin litattafansa amma bai rubuta komai game da rawar da ya taka a cikinsu ba. A ranar 10 ga Yuni, Longstreet ya ba da umurni na Kamfanin A na takwas na William J. Worth na biyu. Ya sake yin yaki tare da sojojin Taylor a yakin Monterrey a watan Satumba na 1846. A lokacin yakin, kimanin 'yan bindigar Mexico 200 sun kori wani rukuni na sojojin Amurka. Longstreet, ya umurci kamfanonin A da B, ya jagoranci wani hari, ya kashe ko ya ji rauni kusan rabin 'yan bindiga. A ranar 23 ga Fabrairu, 1847, an daukaka shi zuwa mukamin laftanan na farko. Janar-in-Chief Winfield Scott ya umarci sashen Worth daga rundunar Taylor kuma a karkashin umarninsa kai tsaye don shiga cikin harin da aka kaiwa babban birnin Mexico na Mexico. An tura rukunin Worth na farko zuwa Tsibirin Lobos. Daga can ya yi tafiyar mil 180 zuwa kudu zuwa birnin Veracruz. Worth ya jagoranci sojojin Scott a cikin hanyar da ta dace a birnin, ya isa can a ranar 9 ga Maris. Scott ya kewaye birnin kuma ya yi ta kai hare-hare akai-akai. Ya mika wuya a ranar 29 ga Maris. Sojojin Amurka sun yi tafiya zuwa arewa zuwa babban birnin. A watan Agusta, Longstreet ya yi aiki a yakin Churubusco, wani gwagwarmaya mai mahimmanci yayin da sojojin Amurka suka matsa kusa da kama birnin Mexico. Rundunar 'yan bindiga ta takwas ita ce kadai karfi a cikin sashen Worth don isa aikin aikin Mexico. Longstreet ya ɗauki banner na regimental a karkashin wuta mai karfi na Mexico. Sojojin sun sami kansu a cikin rami kuma suna iya tsallake kariya ta Mexico kawai ta hanyar tsayawa a kan juna. A cikin gwagwarmayar gwagwarmaya da ta biyo baya, Amurkawa sun ci nasara. Longstreet ya sami ci gaba ga kyaftin din don ayyukansa a Churubusco.