The purpose of the study is to examine how a multimodal composition assignment can be configured to create opportunities for writing development among second language students. In the field of second language writing, where linguistic development has long been prioritized in research and pedagogy, there are concerns that engaging with modes other than language would take away the opportunity to development writing skills. This study intends to address this concern by investigating how second language students interact with language, employ writing strategies, and develop knowledge about writing during a multimodal and a traditional assignment. To this end, I designed two assignments, a traditional written proposal assignment and a video proposal assignment, that were taught by the same instructor in two parallel sections of first-year composition for multilingual students. Five participants were recruited from each section. The data were collected from students’ screen-recordings with think-aloud protocols, writings produced during the two assignments, class observation notes, and semi-structured interviews at the end of the assignments. The findings suggest that students in both groups have engaged in similar ways of translating their ideas into written English with appropriate grammatical, syntactical, and lexical choices. The participants also employed various writing strategies at different stages of the assignments. Students in both groups reported a growing understanding of rhetorical appeals and research-based writing. The video assignment, however, seems to create additional learning opportunities by mobilizing students’ problem-solving skills and a wider range of literacy experiences. At the same time, two students in the video group were observed borrowing online texts verbatim without making attribution, which could be a potentially risky act in the academic context. Based on the findings, I argue that multimodal writing pedagogy should be discussed in relation to the specific design features and learning outcomes. Future research could continue the investigation of students’ multimodal composing processes, with a special focus on the textual borrowing practices across contexts. There is also need for a systematic review that discusses how the concept of multimodal writing is embodied in different assignments, tasks, and genres.