Drawing on a theorized understanding of grammar as a context-sensitive, meaning-making resource for improving writing performance, this study set out to investigate the effectiveness of the grammar support on Korean college students’ EFL composition. The support was provided in the form of an individualized online grammar practice using a website designed for the study, and the grammar points on the website were selected based on the error analysis results of 238 Korean student essays. Forty-eight participants took part in the study for 10 weeks. According to each student’s strengths and weaknesses in grammar skills, the experimental group was assigned the individualized online grammar practice as a follow-up task twice while the control group was not. The statistical analyses indicate a positive effect on writing performance. The survey results illustrate that a majority of the participants deemed the online grammar support helpful and beneficial in writing better compositions. This study is significant in supplying rigorous, theorized evidence for the potential benefits of individualized grammar support to foster development in EFL writing.