Colorectal cancer is the third most common cancer worldwide, and its incidence continues to grow. Approximately one-third of patients with colorectal cancer develop liver metastases during the natural course of disease. Complete surgical resection is associated with very low mortality in colorectal liver metastasis patients, but only a small fraction of colorectal liver metastasis patients fulfill the selection criteria for surgical treatment. We herein describe a high-risk stage-IV rectal carcinoma patient who was initially unresectable according to the National Comprehensive Cancer Network guidelines with a clinical risk score of 4 but received conversion surgery combined with systemic chemotherapy and achieved a favorable long-term clinical outcome (pathologic complete response) of approximately 28 months. Furthermore, serial circulating tumor DNA monitoring using next-generation sequencing provided a comprehensive view of the patient's clinical and pathologic status for better clinical decision support over the course of the disease. The absence of circulating tumor DNA/cells after conversion surgery was correlated with pathologic complete response. This case study not only demonstrated that a curative oncosurgical approach could be considered for high-risk colorectal liver metastasis patients under specific circumstances but also highlighted the role of circulating tumor DNA monitoring to gain further insight into the evolution of a patient's response over time.