Background/Objective: Despite surgery being the primary curative treatment for cancer, patients with compromised preoperative physical, nutritional, and psychological status are often at a higher risk for complications. While various screening tools exist to assess physical, nutritional, and psychological status, there is currently no standardised self-reporting tool, or established cut-off points for comprehensive risk assessment. This study aims to develop, validate, and implement an online self-reporting preoperative screening tool that identifies modifiable risk factors in cancer surgery patients. Methods: This mixed-methods study consists of three distinct stages: (1) Development—(i) a scoping review to identify available physical, nutritional, and psychological screening tools; (ii) a Delphi study to gain consensus on the use of available screening tools; and (iii) a development of the online screening tool to determine patients at high risk of postoperative complications. (2) Testing—a prospective cohort study determining the correlation between at-risk patients and postoperative complications. (3) Implementation—the formulation of an implementation policy document considering feasibility. Conclusions: The timely identification of high-risk patients, based on their preoperative physical, nutritional, and psychological statuses, would enable referral to targeted interventions. The implementation of a preoperative online screening tool would streamline this identification process while minimising unwarranted variation in preoperative treatment optimisation. This systematic approach would not only support high-risk patients but also allow for more efficient provision of surgery to low-risk patients through effective risk stratification.