The current study investigated the role of visual short-term memory, working memory, and declarative memory as individual differences in the earliest stages of vocabulary and syntactic learning in Duolingo-based language instruction. Thirty-eight L1-Greek adults completed memory tasks and engaged in learning Navajo on Duolingo. Subsequently, vocabulary and syntax were assessed respectively via a word recognition, a word translation, and a grammaticality judgement task. Multiple regression analyses revealed an advantage for distributed practice both in vocabulary and syntax, after controlling for amount of practice. Further, declarative memory played a significant role in learning syntax and vocabulary, when measured in a word translation task. Extending the analysis for the first time to app-based environments, the results of the present study confirm the importance of declarative memory and distributed practice in adult acquisition of L2 vocabulary and syntax.