In the present work, the effect of a flow maldistribution on the thermal and conversion response of a monolithic catalytic converter is investigated. To achieve this goal, a combined chemical reaction and multidimensional fluid dynamic mathematical model has been developed. The present results show that flow uniformity within the monolith brick has a significant impact on light-off performance of the catalytic converter. In the case of lower flow uniformity, large portions of the monolith remain cold due to locally concentrated high velocities, and CO and HC are unconverted during the warm-up period, which leads to retardation of light-off. It has also been found that the heat-up pattern of the monolith is similar to the flow distribution profile in the early stage of the reaction. It may be concluded that flow maldistribution can cause a significant retardation of the light-off and, hence, can eventually worsen the conversion efficiency of an automotive catalytic converter.