Background: Variations in the skill profiles of learners have become an important research area in recent years. However, there is a lack of empirical research on this topic in Japan. We conducted three studies to address this gap. Methods: Study 1 investigated the characteristics of the flat and uneven skill profiles of Japanese learners of English using 10 datasets from five standardized four-skill second-language English proficiency tests. Studies 2 and 3 examined the reasons behind learners having these uneven profiles using a convergent mixed-methods approach (Creswell & Plano Clark, Designing and conducting mixed methods research, 2018) that consisted of a questionnaire and an interview, respectively. Results: The results of Study 1 suggested that a flat profile is uncommon, and that various types of uneven profiles exist across datasets. The most frequently observed uneven profiles were as follows: (a) listening, speaking, and writing are lower than reading (LSW<R); (b) speaking and writing are lower than listening and reading (SW<LR); and (c) speaking is lower than the other three skills (S<LRW). The results of Studies 2 and 3 suggested three key reasons, namely, (a) insufficient practice, (b) particular subskills or processes required to accomplish test tasks, and (c) unfamiliarity with test formats and test-taking environments. Conclusions: The results demonstrated the importance of considering uneven skill profiles in L2 research and practice. These results have implications for test development, admission, placement, and pedagogy. For example, skill profile information can benefit admission and placement officers when they make a decision and can aid teachers and administrators in planning remedial instructions.