This study investigates how digital multimodal composing (DMC)—the creation of meaning through the integrated use of modes such as written text, images, audio, video, and animation—supports disciplinary content learning in a content and language integrated learning (CLIL) context. The study is situated in a legal English course for university-level English majors, where students engaged in DMC tasks such as scripting, filming, and editing short explanatory videos on legal topics. Although prior DMC research has largely focused on language instruction, its potential for facilitating subject knowledge development in content-based courses remains underexplored. Drawing on data from eleven student-produced videos created by 32 students, 32 accompanying reflective essays, and records of the composing process, the study examines when and how DMC contributes to content learning. Using thematic analysis and multimodal analysis, the findings show that content engagement varies across stages of the DMC process. Substantial disciplinary learning occurs during topic selection and planning, while scripting promotes the integration and reformulation of specialized knowledge for non-expert audiences. Later production stages emphasize multimodal design and involve less explicit content learning. Overall, DMC supports content learning through conceptual integration, expansion, and recontextualization, highlighting its value for higher education CLIL settings.