The present work describes the design and operation of an autothermal microchannel reactor for portable and distributed hydrogen production via ammonia decomposition. The microreactor consists of an array of alternating catalytic plate channels where the heat for the endothermic ammonia decomposition reaction is supplied in adjacent microchannels via exothermic oxy-fuel combustion of ammonia. The reactor performance was investigated under various operating conditions, such as ammonia flow rate in the decomposition channel, combustible feed flow rate, and fuel equivalence ratios. The best operating conditions were obtained at ammonia flow rate of 0.4 NLPM, combustible feed flow rate of 0.8 NLPM, and fuel-rich operation corresponding to fuel equivalence ratio of 1.2. At these conditions, > 99% NH3 conversion was obtained, and the reactor could generate enough hydrogen for a 45 W-e fuel cell system at an equivalent reactor power density of 1.5 kW(e)/L. Overall, the work reported here successfully demonstrated the feasibility of ammonia decomposition for distributed hydrogen generation and delineated the attainable operating region, results of which could be used further to advance and scale-up autothermal ammonia-fueled microchannel reactors in the kW-range.