Background: The rising global prevalence of obesity and type 2 diabetes is closely linked to excessive consumption of dietary fat and sugar. Although Drosophila melanogaster is a well-established model for diet-induced metabolic dysfunction, most studies examine high-fat diets (HFDs) or high-sugar diets (HSDs) independently, leaving the combined metabolic effects of these major components of Western diets insufficiently characterized. Aim: To compare the metabolic, developmental, and behavioural effects of HFD, HSD, and a combined high-fat/high-sugar diet (HFD+HSD) in Drosophila melanogaster. Methods: Flies were reared on control, HFD, HSD, or HFD+HSD diets. Developmental timing, larval morphology, adult body weight, locomotor activity, and systemic biochemical parameters were assessed. Expression levels of Brummer (bmm) and Insulin-like peptide 2 (ILP2) were quantified using RT-PCR. Data were analyzed using ANOVA with Post-hoc Tukey’s Honestly Significant Difference test in IBM SPSS (Version 23). Results: HSD markedly delayed eclosion (23 vs. 11 days in controls) and significantly reduced larval length (245.48 ± 34.81 µm vs. 724.21 ± 61.28 µm). The combined HFD+HSD diet produced the most pronounced metabolic disturbances, with significant elevations in systemic glucose (male: p = 0.004; female: p < 0.001) and lipid levels. Locomotor performance was also most severely impaired in the HFD+HSD group. Gene expression analysis revealed significant up-regulation of bmm and ILP2, suggesting enhanced lipid mobilization and disrupted insulin signalling consistent with early obesity-associated metabolic dysregulation. Conclusion: Combined exposure to high fat and high sugar induces a stronger metabolic dysfunction phenotype than either diet alone. This integrated Drosophila model offers a powerful platform for investigating the multifactorial mechanisms underlying obesity and related metabolic disorders.