The number of English language learners (ELLs) mainstreamed into regular classrooms continues to increase. Curricular writing standards required by the Common Core State Standards require students to write essays analytically in response to text(s). Many English Language Arts (ELA) teachers may worry about effectively delivering essay writing instruction to their ELL students while still meeting the needs of their proficient English-speaking students. This action research study is a direct response to the concern of two middle level teachers about the writing performance of students in their linguistically diverse classrooms on their state's writing assessment. The study examined the effects of strategy-focused writing instruction on the argumentative essay-writing skills of 47 linguistically diverse seventh-grade students. The students received the strategy-focused writing instruction five days per week for 40 minutes over a 16-week period. Analyses of the data revealed that students' overall writing performance increased significantly from pretest to posttest. Students also made significant gains across the pre- and posttest period in the following two domains: (1) Evidence and Elaboration and (2) Conventions of Standard English, but not on the Purpose, Focus, and Organization domain.