Patients with cancer and diabetes face unique challenges. Limited data are available on diabetes management in patients undergoing concurrent chemoradiotherapy (CCRT), a curative intent anticancer therapy commonly associated with glucocorticoid administration, weight fluctuations and enteral feeds. This retrospective case–control study examined the real-world incidence of acute diabetes-related complications in patients with head and neck cancer receiving CCRT, along with the impact of diabetes on CCRT tolerance and outcomes. Methods: Consecutive patients with head and neck squamous cell or nasopharyngeal cancer who underwent definitive or adjuvant CCRT between 2010 and 2019 at two large cancer centers in Australia were included. Clinicopathological characteristics, treatment complications and outcomes were collected from medical records. Results: Of 282 patients who received CCRT, 29 (10.3%) had pre-existing type 2 diabetes. None had type 1 diabetes. The majority (74.5%) required enteral feeding. A higher proportion of patients with diabetes required admission to a high-dependency or intensive care unit (17.2 versus 4.0%, p = 0.003). This difference was driven by the group who required insulin at baseline (n = 5), of which four (80.0%) were admitted to a high-dependency unit with diabetes-related complications, and three (60.0%) required omission of at least one cycle of chemotherapy. Conclusions: Patients with diabetes requiring insulin have a high risk of acute life-threatening diabetes-related complications while receiving CCRT. We recommend multidisciplinary management involving a diabetes specialist, educator, dietitian, and pharmacist, in collaboration with the cancer care team, to better avoid these complications.