ABSTRACT Background Informal digital learning of English (IDLE) contributes significantly to language development, and teachers play an essential role in supporting IDLE, especially in the K‐12 context. Yet, insights into K‐12 teachers' instructional practices in support of IDLE and influencing factors thereof are limited. Objectives Considering that professional learning influences teachers' instructional decision‐making, this study aimed to examine how formal and informal sources of professional learning related to teachers' instructional practices in supporting IDLE. Methods We adopted a mixed‐methods sequential explanatory design that involved a questionnaire and semi‐structured interview. A total of 1760 K‐12 language teachers completed an online survey, and nine teachers were recruited for follow‐up interviews to explain and elaborate on the quantitative findings. Results and Conclusions SEM analyses of survey responses revealed that informal professional learning sources fully mediated the association between formal professional development and teachers' instructional strategies in support of IDLE. Informal professional learning sources had varying impacts, with interaction with students and reflection emerging as the most significant contributors. However, their influence varied across different types of instructional practices. The findings emphasise the need for utilising different informal professional learning sources to enhance K‐12 teachers' attention and engagement in supporting students' IDLE.