In this work, activated carbon sulfonic acid was prepared from the reaction of activated carbon and chlorosulfonic acid in chloroform at reflux conditions and characterized using X-ray powder diffraction (XRD) spectrum, infra-red (IR) spectrum, field emission scanning electron microscopy (FE-SEM) images and energy dispersive X-ray spectroscopy (EDS). Benzimidazole was prepared in excellent yields through the multicomponent condensation reaction of 1,2-phenylenediamine with aryl aldehydes in the presence of sulfonic acid-functionalized activated carbon (AC-SO3H), as an active catalyst, under solvent-free conditions. According to the optimized variables, the best reaction conditions for preparing benzimidazole were found to be: 0.02 gram of catalyst in solvent-free condition at 30 Min. and at 75 °C. To demonstrate the stability and durability of the catalyst, the yields of five successive runs with recovered catalyst were reported, showing no significant change in the obtained yields. Ultimately, the synthesis of benzimidazoles was achieved using an efficient, simple, environmentally benign, inexpensive and economic approach in the presence of AC-SO3H catalyst.