This study employed a mixed-methods approach to explore the attitudes of second language (L2) learners and L2 teachers toward the use of learners' first language (L1) in L2 classrooms. Interview data were collected from Japanese college students (n = 91) and their English teachers (n = 9) to identify salient themes related to the reasons for supporting or opposing teachers' use of L1. Additionally, data on the students' English proficiency and experiences with English as a lingua franca (ELF) communication were collected to determine whether these factors influence their views. The results revealed that all the teacher participants support and/or actually use L1 out of necessity. In contrast, approximately 60% of the student participants expressed negative views regarding L2 teachers' recourse to L1. Logistic regression analysis showed that these attitudes are influenced by both students' past experiences with ELF communication and L2 proficiency. Although both groups recognized the benefits of L1 use, the students' compelling arguments against L1 use in L2 classrooms underscore the need for further scholarly attention to the extent and function of L1 use as a pedagogical tool and resource.