The knowledge of English idioms is essential for advanced English learning and application, particularly for English-as-a-foreign-language (EFL) learners. While studies have shown the benefits of mobile-assisted language learning (MALL) to facilitate English learning, few studies have examined the factors that may influence MALL in light of motivation and anxiety, let alone the behavioral patterns of learners with different learning motivation and learning anxiety levels. Therefore, in this study, a mobile application named My English Idiom Learning Assistant (MEILA) rooted in the theory of Associative Fluency was developed to examine the relationships among learning outcomes, motivation, anxiety, and learning behaviors in English idiom learning. The participants were 59 students from two Freshman English classes in one private university in Central Taiwan. In this 5-week implementation, the students were engaged in 3-week self-study learning, where they were encouraged to watch 15 idiom-related animations and videos each week and make and record English sentences in MEILA. Multiple data sources were collected in this mixed-methods research, including pre- and post-test of English idiomatic understanding, two questionnaires (English learning motivation and English learning anxiety), and interviews. The results from the tests revealed the positive affordances of MEILA for facilitating English idiom learning among Taiwanese college students. The analysis of students' behavioral patterns showed that students with different levels of motivation and anxiety (high motivation/high anxiety, high motivation/low anxiety, low motivation/high anxiety, low motivation/low anxiety) exhibited distinctive learning behaviors. Most students were positive about the use of MEILA for English idiom learning. The current findings provide recommendations to instructional designers who wish to optimize MALL for benefiting learners with different needs.