Students in the United States continue to rank lower on math and science achievement tests as compared to their international counterparts. In general, this lack of achievement, especially when compared globally, is a nationwide concern and may be linked to larger problems of sub-standard math performance such as individual earning capability, economic growth and income distribution. The specific problem addressed was the continued substandard scores in standardized mathematics achievement tests, and overall lower rankings in international tests, of schools in the Guam Public School system. The purpose of the qualitative, explanatory single case study was to examine how teachers perceived and integrated writing into the math curriculum and how this influenced student cognition to improve student mathematical thinking. Flavell’s metacognitive theory served as the theoretical framework. The participants included 10 middle and high school mathematics teachers in the Guam Public School District. Braun and Clark’s six-step thematic analysis process was used to analyze data. Three themes emerged from analysis: Writing-to-learn is a formative assessment tool in enhancing mathematical thinking, writing is an effective metacognition instructional tool and open discussion methods facilitate students’ writing entries. Results indicated that metacognition can be a valuable tool to improve critical thinking and comprehension in math classes. Educational researchers may determine that a learning strategy has a positive impact on student learning; without the commitment of teachers, there is a strong likelihood that it will not be utilized effectively or even at all.