Ni/Mg/Al catalysts with different atomic ratios were synthesized and tested for simultaneous H2-rich syngas recovery and toxic hydrogen cyanide (HCN) removal from polyurethane cushion waste. The waste decomposition products were converted into H2-rich syngas, and toxic HCN was removed using the Ni/Mg/Al catalysts. The catalyst composition drastically affected the ease of reduction to metallic Ni. Easily reduced catalysts effectively increased the syngas yield with redNi1.0Mg2.1Al6.0, giving the maximum syngas yield (1925 mL/g sample). Moreover, smaller Ni particle sizes (high dispersion) resulted in more effective HCN decomposition. Thus, red-Ni1.0Mg4.9Al4.1 gave the lowest HCN yield (0.9 wt %) and concentration (0.5 vol %). Durability testing revealed a decrease in the catalytic activity with repeated use (first cycle, 1497 mL/g sample; fifth cycle, 1148 mL/g sample). However, regeneration improved the gas yield (1515 mL/g sample) owing to the redispersion of the aggregated Ni particles. These results reveal the potential for the continuous use of catalysts, advancing the possibility of recycling polyurethane waste.