Thiazolium salt pre-catalysts have been immobilized on silica and monolithic polystyrene and their activity was tested under batch conditions in three model umpolung reactions, namely the benzoin condensation of benzaldehyde, the acyloin-type condensation of biacetyl, and the Stetter reaction of biacetyl with trans-chalcone. A prerequisite of the study has been the utilization of environmentally benign water and ethanol solvents. After having established the higher performance of polystyrene monolithic thiazolium carbene catalysts, their effectiveness has been tested under the flow regime by fabricating the corresponding monolithic microreactors (pressure-resistant stainless-steel columns). Importantly, it has been demonstrated by a brief substrate scope study that the polymeric matrix and the continuous flow regime synergistically contribute to preserve the activity of the carbene catalysts over time, thus permitting the long-term operation (up to 7 days) of the prepared monolithic reactors for the production of valuable compounds via the umpolung strategy.