Necrotising fasciitis (NF) is a rapidly progressing, life-threatening infection with mortality rates that are exceedingly high. Despite the notably high risks of developing NF in patients with diabetes mellitus (DM), factors associated with mortality in this population are poorly understood. Therefore, to determine at-risk patients and to improve overall clinical outcomes via hastening management, the objective of this systematic review is to determine what factors are associated with mortality for diabetic patients with NF. This systematic review followed the PRISMA guidelines. Patient data pertaining to outcomes and surgical management were extracted, and mortality rates were evaluated. Studies were assessed for quality using the Alberta Heritage Foundation for Medical Research (AHFMR) and Risk of Bias tool. A total of 25 studies were reviewed, covering 7879 patients with NF and DM with a 23.5% mortality rate. The most prevalent comorbidities among those who died included chronic kidney disease (15.95%), hypertension (9.42%) and obesity (9.02%). While limbs were the most common location for the disease, NF in the trunk and groin regions showed the highest mortality rates at 62.07%. Among those who died, common complications were acute renal failure (13.41%), pulmonary issues (20.41%) and septicaemia (12.80%). Mortality rates by surgical management were fasciotomy (42.9%), surgical debridement (40.68%) and amputation (9.09%). Mortality was comparable between patients with NF and DM (23.54%) and those with NF alone (23.61%). Although DM may not independently increase mortality, it can worsen outcomes when combined with other comorbidities, indicating a need for clearer clinical guidance.