In this paper, we report on the development and implementation of a web-based mobile application, "Grammar Clinic," for an ESL writing class. Drawing on insights from the interactionist approach to Second Language Acquisition (SLA), the Noticing Hypothesis, and mobile-assisted language learning (MALL), "Grammar Clinic" was designed as a series of outside-class grammar exercises in the format of sentence-level error identification and correction. 19 intermediate level ESL students at a large Midwestern American university used this mobile application in an English academic writing class during a 16-week semester. A mixed methods approach was chosen to investigate (a) the effect of "Grammar Clinic" on the efficacy of ESL students' self-editing as well as (b) their perceptions of this mobile application as a tool to improve their English writing. Our analyses show that students' performance on "Grammar Clinic" assignments reflects their progress in self-editing. This progress is seen in the positive correlation of the students' "Grammar Clinic" performance with their gains on a grammar post-test, an increase of self-editing corrections, and a reduction in errors in the final drafts of two major paper assignments. The impact of this web-based mobile application was documented using qualitative data that considered the students' use of and perception of "Grammar Clinic."