Mobile-based extramural English (EE) activities, including watching movies, playing digital games, and listening to music outside the classroom, have played an important role in increasing learners' interest in English as a foreign language (EFL). This mixed-method design study aimed to investigate the role of EE activities in the vocabulary development of intermediate EFL learners. The study included a group of 60 learners, including 30 male and 30 female students from two language institutes. A pretest, posttest, and delayed posttest were used. The experimental group utilized WhatsApp to receive electronic vocabulary instruction four days per week for two months as part of the EE activities. In contrast, the control group received classroom instruction using the conventional approach typically employed in EFL contexts for teaching the same textbook vocabulary. A semi-structured interview was conducted to investigate the participants' beliefs regarding their preference for EE-based activities. The results of the EG group revealed that EE-based activities significantly impacted the students' vocabulary learning (p < 0.001). The findings also revealed no significant differences in vocabulary knowledge between male and female students after using EE activities (p = 0.34). The themes that emerged from the qualitative phase suggested that participants preferred EE-based activities as they can result in less mental stress, more vocabulary learning, improved conversation, and improved pronunciation and grammar. Also, the participants preferred various mobile-based EE activities, including watching movies, listening to music, playing multiple games, watching cartoons, and focusing on movie subtitles, which resulted in their vocabulary development. The findings call for language educators to integrate more virtual and online-based EE activities to supplement traditional vocabulary teaching practices.