Ammonia decomposition presents a critical pathway for hydrogen production and storage by enabling ammonia as a carbon-free hydrogen carrier; however, catalyst deactivation and inefficient nitrogen activation remain key challenges. Herein, we report a defect-engineered ceria-supported ruthenium catalyst, developed by integrating atomic-scale dispersion of Ru with controlled lattice modification of the support. Ruthenium is stabilised as isolated atoms and sub-nanometre clusters on a praseodymium–cobalt co-doped ceria matrix synthesised via an optimised sol gel route, maximising accessible active sites while suppressing sintering. The incorporation of aliovalent co-dopants into a ceria matrix leads to a significant increase in oxygen vacancy concentration and electronic structure, resulting in modified metal–support interactions and enhanced redox capability. These materials' level modifications facilitate N–H bond activation, leading to near-complete ammonia conversion under the investigated conditions. The optimised RC-Pr0.3Co0.2Ox catalyst outperforms undoped ceria and benchmark 1 wt% Ru–CeO2 in both activity and long-term stability. The results highlight defect-mediated metal–support synergy as an interdisciplinary materials design strategy for advancing ammonia cracking catalysts toward integration in solid oxide fuel cell–based, carbon-neutral energy systems.