This study examined the variables related to US immigrants' long-term attainment in English, their second language (L2), and their native language (L1). For 44 Mandarin–English bilinguals, with increasing age of arrival (AOA) in the United States, their accuracy in L2 grammaticality judgment tasks decreased and accuracy in an L1 grammaticality judgment task increased. Moreover, both AOA in the United States and mothers' English proficiency uniquely predicted a significant proportion of the variance for bilinguals' L2 proficiency. Finally, as a group, 72 speakers of three Asian languages showed lower levels of L2 proficiency and stronger AOA effects on the task performance than 32 speakers of six European languages. These differences in language proficiency were associated with differences in language use, language learning motivation, and cultural identification between the two groups. These findings suggest that L2 acquisition in the immigration setting is a complicated process involving the dynamic interactions of multiple variables.