U.S. higher education is increasingly linguistically and culturally diverse (NCES, 2023), bringing L1, Generation 1.5, and L2 undergraduates with differing English proficiency levels into mandatory First-Year Composition (FYC) courses. Most research on L1, L2, and Generation 1.5 writers was conducted over thirty years ago and served as the foundation for the current structure of FYC courses. This body of work needs to be revisited considering present-day classroom realities. More recent research remains scarce but has found that mainstream FYC classes can overlook multilingual students’ specific language needs (e.g., grammar support) and, as a result, these students progress more slowly than their native-speaking counterparts (Eckstein & Chang, 2022). This dissertation aims to build on and extend previous findings by providing a more nuanced understanding of writing development among L1, L2, and Generation 1.5 students in mainstream (ENGL 1101) and multilingual (ENGL 1101m) courses. This study included forty participants: 27 from ENGL 1101 and 13 from ENGL 1101m, representing three language groups—L1 (26), Generation 1.5 (9), and L2 (5). A mixed-methods approach combined pre- and post-test quantitative analysis of writing development with qualitative exploration of students’ course experience. Quantitative data included 29 indices of fluency, lexical and syntactic complexity, accuracy, and global writing features, supplemented by student surveys. Qualitative data were drawn from course documents, teacher and student interviews, classroom observations, and seven case studies of multilingual students’ writing development trajectories. Findings show that overall, students demonstrated significant improvement in fluency and global writing features. Measures of accuracy and syntactic complexity, and most lexical complexity measures (except CTTR), did not show significant gains. Generation 1.5 students exhibited the greatest growth in fluency and global features, while L2 students showed the least development in total scores and source use. Students in the multilingual course showed higher gains in word fluency and global writing scores, whereas mainstream students demonstrated slightly greater progress in accuracy and language use. Case studies revealed that writing development is individual, non-linear, and complex. The results suggest the need for further investigation into differences across global writing features among language groups. Pedagogical and administrative implications are discussed.