Content writing contributes to students' positive learning and outcomes. Mathematical-content writing has been shown to positively support students' mathematical reasoning, but it is challenging for many students with and without learning disabilities. This study explores proof-of-concept evaluation to extend self-regulated strategy development, an evidence-based practice in writing, to mathematical content-specific writing. Twenty-seven students participated in this pilot study, which included seven students with disabilities. The intervention consisted of six lessons that taught students a strategy to self-regulate written responses to open-response mathematics problems. Results indicate that mathematics fact fluency correlated with mathematical writing quality and demonstrated to be a potential predictor for mathematical writing performance. Mean scores indicated that students in the treatment group outperformed students in the control group, although the difference was not statistically significant. Results support the proof-of-concept that teaching students a self-regulated strategy for mathematical writing may improve student outcomes, although future research is needed with a greater number of participants that receive longer dosage of the intervention to more confidently support this claim. The implications for future research and practice were discussed.