Most U.S. colleges and universities expect students to improve their writing ability by taking first-year composition (FYC) courses. In such courses, non-native English (L2) writers with diverse language backgrounds study alongside their native English (L1) speaking peers. However, it is not clear how different these populations are in terms of their language development over time, leaving questions unanswered about whether L2 writers develop more or less than L1 writers in an FYC curriculum. To investigate, we compared 75 L1 and L2 students' written accuracy, fluency, and lexical and syntactic complexity over the semester of an FYC course. Data showed that L2 students had significantly higher rates of language error and less fluent and lexically complex writing compared to L1 writers. Moreover, L2 student writing became less grammatically accurate over 14 weeks despite showing greater fluency and syntactic complexity. These results suggest a need for plurilingual pedagogies in FYC that embrace diversity and inclusion while also providing L2 writers with instruction on socially powerful and dominant linguistic forms.