While the positive role of engagement in students' learning achievements has been well documented, limited studies have examined whether or how learner engagement would vary with their achievements over time, especially within an unstructured digital learning context. This paper using a purposive sampling method reported an exploratory study on how English as a foreign language (EFL) undergraduate students with lower and upper proficiency levels (LP and UP) behaviorally engaged in their self-initiated mobile-assisted English learning (MAEL) beyond the classroom. Participants were senior-year undergraduate students from universities in central China. Quantitative analysis based on 409 questionnaires showed that there were significant differences in both how long and how often students participated in MAEL across proficiency levels, with LP learners investing more time on a more frequent basis than UP learners. However, there was no significant effect of language proficiency on the number of students' MAEL resources employed. Besides, EFL proficiency was also found to play a mediating role in students' learning preferences despite their common interest in receptive learning practices. Qualitative data from 36 follow-up semi-structured interviews revealed students' cognitive and emotional rationales behind these learning behaviors. This study might offer some fresh insight into the complexity of foreign language learner engagement with self-initiated mobile-assisted learning in Chinese higher education.