Objectives: Fournier’s gangrene is an aggressive, rapidly progressing, and life-threatening necrotizing fasciitis of the perineal and genital regions. Various scoring systems have been developed for predicting survival and prognosis in Fournier’s gangrene. This retrospective study aimed to evaluate the effectiveness of the newly developed Fournier’s gangrene mortality index (FGMI) in predicting mortality associated with Fournier’s gangrene. Methods: The study included patients over the age of 18 years who were followed-up with a diagnosis of Fournier’s gangrene in the general surgery clinics of three different hospitals in Şanlıurfa province between 2014 and 2024. The patients included in this study were divided into two groups: deceased (n = 20) and surviving (n = 149). In FGMI, the parameters used were age, creatinine level, albumin level, lymphocyte percentage, and neutrophil-to-lymphocyte ratio. Based on the total score and risk assessment, <5 points were categorized as low-to-moderate mortality risk and ≥5 points as high mortality risk. Results: A total of 169 patients with a diagnosis of Fournier’s gangrene were included in the study; 87 were men (51.48%). The median age of all patients was 53 (40–63) years; 20 patients (11.8%) died. The Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) score did not show a statistically significant difference between the deceased and surviving groups (p = 0.5). Compared to the survivors, the deceased had higher neutrophil counts, neutrophil percentages, neutrophil-to-lymphocyte ratios, platelet-to-lymphocyte ratios, and C-reactive protein-to-albumin ratios, whereas lymphocyte counts, lymphocyte percentages, eosinophil counts, eosinophil percentages, monocyte counts, and monocyte percentages were lower, and these differences were statistically significant. According to receiver operating characteristic (ROC) analysis, the ROC-area under the curve for predicting mortality based on an FGMI score of ≥5 was 0.88 (95% CI: 0.80–0.95) with a sensitivity of 90% and a specificity of 70% (p < 0.001). Univariate risk analysis was performed, and the odds ratio revealed that mortality risk in patients followed-up for Fournier’s gangrene with a FGMI score of ≥5 was 20 times higher (4.48–90.91) (p < 0.001). Conclusions: The results reveal that the FGMI score is a scoring system that can predict mortality at the initial clinical presentation of patients with Fournier’s gangrene. Another important finding of the present study is that the LRINEC score was not sufficiently effective in predicting mortality.