The valorization of cow manure (CM), as bio-waste, under a CO2 atmosphere could be an attractive strategy for tackling the environmental problems related to waste management and CO2 emission and producing valuable syngas. For this purpose, highly loaded Ni–Al2O3 catalysts with alkaline-earth metals (Mg and Sr) were synthesized and applied to the gasification of CM under CO2. The lowest yields of bio-oil (16.98 wt %) and coke (0.34 wt %) and the highest yield of syngas (55.09 wt %) were obtained from the catalytic decomposition of hydrocarbons when Sr was incorporated into Ni/Al2O3 (SN-AO). The highest selectivity for H2 (34.23 vol %) and CO (37.16 vol %) were obtained applying SN-AO followed by Mg-promoted Ni/Al2O3 (MN-AO) and Ni/Al2O3 (N-AO) catalysts. With increasing gasification temperature from 750 °C to 850 °C, the syngas yield (from 55.09 to 70.17 wt %) and H2 concentration (from 34.23 to 38.03 vol %) increased considerably because of the endothermic gasification process. The yield and selectivity of syngas (H2 and CO) increased under CO2 compared to those obtained under N2, indicating the high potential of CO2 for the thermal decomposition and dehydrogenation of the volatile matter.