As a more compact, affordable and efficient alternative to traditional catalytic converters, catalytic hollow fibre-based reactors have significant potential in addressing the high nitrogen oxides (NOx) emission associated with the combustion of green ammonia. In this work, the performance of a series of manganese (Mn)-based catalysts supported on three different carbon xerogels during the ammonia-selective catalytic reduction (NH₃-SCR) reaction was investigated in a packed bed reactor configuration under typical vehicle exhaust gas conditions. The best catalyst candidate Mn-CX, which was associated with the highest NO conversion (i.e. 24% at 225 °C) and highest nitrogen selectivity (i.e. 85% at 225 °C), was deposited in a 7-channelled hollow fibre substrate via a combined sol-gel and incipient wetness impregnation method. At 225 °C and 1 atm, the performance of the hollow fibre-based reactor was enhanced by a factor of four compared to the packed bed reactor (i.e. r_O2 = 3300 mol_O2∙m⁻³∙h⁻¹∙g_cat⁻¹ and r_O2 = 810 mol_O2∙m⁻³∙h⁻¹∙g_cat⁻¹). The superior performance of the hollow fibre-based reactor is attributable to the unique morphology of the hollow fibre substrate, which lends itself to minimised mass transfer limitations. The markedly improved performance of the hollow fibre-based reactor underlines its potential as a technically and economically feasible solution to mitigate the high NO_x emissions associated with ammonia combustion. The identification of the catalytic hollow fibre-based reactor as a viable exhaust gas after-treatment technology for green ammonia-fuelled engines, addresses a significant barrier facing the adoption of green ammonia as a carbon-free, future fuel, thereby facilitating the transition to a decarbonised transport sector.