We evaluated external and internal sources of variation in second language (L2) and native language (L1) proficiency among college students. One hundred and twelve native-English L2 learners completed measures of L1 and L2 speaking proficiency, working memory, and cognitive control and provided self-ratings of language exposure and use. When considering learner-external variation, we found that more frequent L2 exposure predicted higher L2 and L1 proficiency, while earlier L2 exposure predicted higher L2 proficiency, but poorer L1 maintenance. L1-L2 distance limited crosslinguistic transfer of print-to-sound mappings. When considering learner-internal variation, we found that L1 and L2 proficiency were highly correlated and that better working memory, but not cognitive control, accounted for additional variance in L2 and L1 proficiency. More frequent L2 exposure was associated with better cognitive control.