This mixed-methods study examined the association between informal digital learning of English (IDLE) and English as a Foreign Language (EFL) learners’ vocabulary knowledge (VK). A total of 365 EFL learners, including 172 males and 193 females, participated in the quantitative part, while 24 EFL learners (12 males and 12 females) participated in the qualitative part. The participants completed the Informal Digital Learning of English Scale (IDLES), an English vocabulary test, and wrote a reflective statement. Pearson correlation analysis, multiple regression analysis, and thematic coding analysis were employed to analyze the collected data. The findings revealed a significant positive correlation between IDLE and the EFL learners’ VK. Additionally, the results indicated that the EFL learners’ VK was influenced by all dimensions of IDLES, including IDLE-enhanced benefits, IDLE practices, others and support, authentic L2 experience, resource and cognition, and frequency and device. Moreover, four overarching themes emerged from the qualitative findings: having authentic learning experiences, receiving support from others, benefiting from quantity and diversity, and being motivated and confident. The findings evidenced that IDLE is an effective way of enhancing EFL learners’ VK as it can provide them with exposure to authentic and diverse English learning resources and activities beyond traditional classrooms.