This work evaluated the stimulatory effect of zinc (provided as ZnSO and ZnCl; 50, 70 and 100 mg/L Zn) supplementation on biogas (methane) production, while co-digesting a mixture of food waste and domestic wastewater (0.183, v/v) in an upflow anaerobic sludge blanket reactor operated under mesophilic condition at pH 7.6 and 10 days of hydraulic retention time. The intermittent feeding mode (48 h feed and 48 h feedless) was applied to avoid the reactor failure due to the accumulation of short and long chain fatty acids (SCFAs and LCFAs). With the increasing zinc supplementation from 50 to 100 mg/L Zn, chemical oxygen demand removal efficiency and methane yield increased by 10 and 30–65%, respectively, compared to the control without zinc supplementation. This improvement was considered mainly attributed to the higher conversion of organic matter into methane since this microelement is essential to many enzymes involved in the anaerobic reactions. Regardless of the Zn concentration, the total SCFAs accumulation was decreased, and together with the decrease of acetate concentration with the increase of zinc supplementation, SCFAs other than acetate might have been converted directly to biogas (methane) through pathways different from ordinary hydrogenotrophic and acetotrophic methanogenesis. There were statistically significant differences (p < 0.05) in the effluent total LCFAs concentration, regardless of the influent supplemented with different zinc concentrations. The disappearance of the unsaturated ones (oleate and linoleate) after the microelement supplementation could be related to the contribution of biological and physical (precipitation) removal.