Healing process of the gaps and cracks in the structure of geopolymer binders, which have emerged as greener alternatives compared to their traditional Portland cement counterparts, is of great importance in terms of a long economic life. The application of the microbial induced carbonate precipitation (MICP) method, which is the most striking of the various healing techniques applied to building materials, on geopolymer composites has been limited to a few successful studies. At this point, it is considered to be an important step to examine the effects of addition of bacteria on the mechanical and durability characteristics of geopolymer composites in detail. Therefore, this paper was designed to examine the effect of the usage of Bacillus subtilis on the mechanical and durability performance of ground blast furnace slag (GBFS)-based geopolymer mortar (GPM) specimens. In the GPM specimens activated with Na₂SiO₃, the total liquid/binder ratio as 0.55 and the binder/fine aggregate ratio as 1:2 was kept constant. Bacterial cultures in liquid form prepared at different concentrations (10⁹ and 10⁷ CFU/mL) were added directly to the Na₂SiO₃ at 0, 1, 2 and 3% by weight of GBFS. GPM samples, which were prepared in seven different groups in total, were kept in three different curing mediums (precipitation medium, water and ambient conditions) from 7th day to the 28th day. After the curing period was over, the compressive strength, electrical resistivity, sulfate and acid resistance, capillary water absorption, splitting tensile strength and permeability properties were investigated on the GPM samples. In addition, the above-mentioned test results were confirmed by the microstructural analyzes performed. Experimental findings revealed that the optimum bacterial concentration and bacterial dosage values were 10⁷ CFU/mL and 3%, respectively, in terms of both mechanical properties and durability performances. On the other hand, it was observed that all of the GPM specimens cured in precipitation medium (PM) and produced using bacteria had superior performances compared to their counterparts cured in water and ambient conditions. This situation clearly demonstrated that an effective self-healing process that will occur in GPM samples produced with the addition of bacteria directly could only be possible by creating a curing environment containing urea and calcium.