Much research in Mathematics instruction has focused on collaborative learning and differentiated instruction. However, very little research in the Philippines focused on utilizing writing activities as an instructional intervention. Even in Mathematics, a subject grounded in computations, this can be beneficial. By explaining how a problem is solved, or why a solution is erroneous, students will learn how the concepts may be applied in a deeper sense. Given the pandemic's limits and the Philippines' low-performance ratings in international assessments, there is a pressing need to develop innovations to continue learning. Hence, this study investigated whether writing activities are effective in improving academic achievement in mathematics classrooms. Using an explanatory sequential mixed methods design, the study involved selected Grade 8 students in a public school in Pasig City. The results of the quantitative data showed a significant difference in the pre-test and post-test scores of the experimental group as compared to the control. This was supported by the qualitative data which revealed that writing activities help understand the topics, remember concepts, and serve as a reviewer before an assessment. Overall, the study suggests that writing activities as an intervention in mathematics are effective in improving the student’s academic achievement.