Fe or Cu oxides and their composites are cost-effective yet seldom used in DeNOx because they deactivate easily. Doping/supporting enhances activity but increases synthesis complexity and cost. Here, we present a γ-Fe2O3-CuO composite oxide with a high synergy created through co-oxidation, which upon magnetization, exhibits further enhanced DeNOx activity. Detailed quasi in-situ XRD and in-situ DRIFTS analyses indicate that in Fe-Cu composite oxides, iron species inhibit the full reduction of Cu2+ to Cu0, while copper counters the formation of inactive α-Fe2O3, contributing to the γ-Fe2O3-CuO composite’s enhanced stability during reactions. Fascinatingly, pre- and post-magnetization XPS and in-situ DRIFTS analyses demonstrate the magnetic field’s role in promoting the chemisorption of both oxygen and NO, thereby enhancing DeNOx efficiency. This research not only achieves the synergistic coupling of copper and iron oxides and sheds light on the deactivation processes of their pure species but also serves as a pioneering exploration of magnetic manipulation in catalyst design, laying a foundation for future technological advancements.