Pumpkin leaves (Cucurbita maxima L.) are an underutilized leafy vegetable that represents a rich source of nutrients and bioactive compounds. This study assessed the effects of four common household cooking methods—boiling, steaming, microwaving, and stir-frying—on the nutrient composition, amino acid profile, phenolic compounds, carotenoids, antioxidant potential, antinutritional factors, and α-amylase inhibitory activity of young pumpkin leaves. Steaming resulted in the highest apparent retention of phenolics, flavonoids, and carotenoids. Microwave treatment also maintained relatively high levels of phytochemicals, whereas boiling caused the greatest losses of bioactive compounds. Steaming also showed the strongest α-amylase inhibitory activity. Boiling, on the other hand, was the most effective method for reducing antinutritional compounds. The results demonstrate that culinary processing significantly affects the nutritional and functional quality of pumpkin leaves. Collectively, the results suggest that steaming was the most effective method for preserving nutrients and bioactive compounds, whereas boiling was more effective in reducing antinutritional compounds. These findings provide practical evidence-based recommendations for optimizing household cooking methods according to specific nutritional objectives.