Achieving and maintaining mutual intelligibility between interlocutors has been emphasized in the second language (L2) pronunciation acquisition and pedagogy. Although previous studies identified some listener background factors that contribute to enhancing non-native speakers’ (NNS) speech intelligibility, little studies investigated linguistic and non-linguistic background factors together. Hence, the present study investigated the combined effect of listeners' language background and L2 English teaching background factors by using mixed-methods approach. This study conducted intelligibility transcription task of 60 listener participants who were divided into 4 listener groups according to listeners’ L1 and L2 teaching experience. The study found that Koreans’ speech was more intelligible to L1 Korean teachers than to native English (NE) non-teachers when both listeners’ language and L2 teaching experience were considered. In addition, qualitative analysis of the transcription task showed that L1 Korean teachers demonstrated more accurate perception than NE listeners to transcribe certain words or phrases uttered by some L1 Korean speakers. The present study found additional evidence for complex and subtle nature of mutual intelligibility, which reacts sensitively towards multitudes of listener background factors. It also found the positive effect of L2 English teaching experience on foreign-accented speech intelligibility.