This study investigated the effectiveness of the Genre-Based Writing Task (GEB-WRIT) intervention in enhancing the writing proficiency of Grade 12 learners at Sto. Niño National High School in Talaingod, Davao del Norte, Philippines. Specifically, it aimed to determine the level of students’ writing proficiency before and after the intervention and to gather insights regarding their experiences with the GEB-WRIT strategy. Utilizing a quantitative-descriptive research design supplemented with interviews and focus group discussions, the study assessed the writing performance of 21 Grade 12–Quezon students across pre-test, intervention, and post-test phases. Additionally, 10 students participated in the qualitative component. Pre-test results indicated very low writing proficiency, with a mean score of 114. Following the intervention, post-test scores demonstrated a significant improvement, with a mean of 236, corresponding to a high level of writing proficiency. A paired t-test confirmed a statistically significant difference (p < .001) and a very large effect size (Cohen’s d = 3.49). Qualitative findings supported these results, revealing improved confidence, reduced apprehension in writing, stronger understanding of genre-specific structures, and appreciation for the structured approach. Participants also suggested extending the program’s duration and incorporating group work to foster collaboration. The findings affirm that the GEB-WRIT intervention is an effective strategy for developing writing proficiency and learner engagement in senior high school contexts.