Display omitted •Embarks on novel nanocomposite electrocatalyst syntheses from biomass and transition metal oxide.•The nanocomposite demonstrates high electrocatalytic activity and durability for the hydrogen evolution reaction.•Biowaste-derived carbon promotes fast charge transfer and well-dispersed sulfur-modified metal oxide creates abundant active sites.•Sulfur incorporation modulates the adsorption energy of water molecules on the catalyst surface.•The energy barrier for water dissociation is lowered and the activity is enhanced over the sulfur-incorporated metal oxide surface. In the pursuit of a more sustainable future and to combat the environmental impact of fossil fuels, green hydrogen production through water electrolysis is gaining traction. While noble metals currently dominate electrocatalyst design, their expense limits widespread adoption. This study proposes a solution by outlining a method for designing efficient and affordable electrocatalysts based on non-precious metals. Herein, we introduce a novel S-incorporated Ni2O3 supported on biowaste-derived activated carbon (S-Ni2O3@AC) synthesized from pumpkin shells. This facile and sustainable approach utilizes readily available resources. The S-Ni2O3@AC nanocomposite demonstrates excellent HER performance with low overpotential and high durability. This is attributed to a high surface area of the composite, fast charge transfer, optimal hydrogen adsorption, and lowered energy barrier for water dissociation facilitated by sulfur incorporation, which is demonstrated by density functional theory (DFT) calculations. This research paves the way for cost-effective and sustainable hydrogen evolution electrocatalysts.