Background <p>Preoperative colonoscopy is recommended for colorectal cancers to exclude synchronous tumors.&#xa0;However, data are lacking regarding this recommendation for appendiceal mucinous neoplasms. This study evaluated whether preoperative colonoscopy in patients with appendiceal mucinous lesions would reveal a significant finding, modify the operative plan, or be associated with adverse events.</p> Methods <p>This mixed-methods retrospective cohort study included all mucinous appendiceal neoplasms between 2017 and 2024 at a tertiary care hospital.</p> Results <p>Overall, 100 patients were included: 15% presented with acute appendicitis, 31% presented with an appendiceal mucinous lesion, and 54% presented with pseudomyxoma peritonei. Preoperative colonoscopy was performed in 83% of patients, 73.4% of which were normal, 19.3% showed benign polyps, 3.6% demonstrated caecal base invasion, 2.4% showed appendicular orifice invasion, and 1.2% had extrinsic caecal compression. No synchronous colorectal tumors were identified. When caecal invasion was demonstrated on colonoscopy, it had already been identified on preoperative computed tomography scan. The operative approach was never modified based on the colonoscopy results. There was no difference in discovery of tumor perforation between patients who underwent colonoscopy and those who did not (85% vs. 75%; <i>p&#xa0;=&#xa0;</i>0.286). Longer time to surgery for those undergoing preoperative colonoscopy (325.9 vs. 116.1 days; <i>p</i>&#xa0;=&#xa0;0.034) reflected the need for thorough work-up, neoadjuvant therapy, specialist consultations, and tertiary-care center referrals for patients with advanced appendiceal mucinous neoplasms.</p> Conclusions <p>Preoperative colonoscopy demonstrated a low yield, a limited capacity to modify operative management, and was not associated with a higher probability of tumor perforation. Colonoscopy may be safely postponed postoperatively in select patients.</p>