This paper aims to investigate the mechanical performance of steel fiber-reinforced geopolymer concrete made with fly ash and ground granulated blast furnace slag as blended aluminosilicate source material. To activate the binding phase, combinations of sodium silicate (SS) and sodium hydroxide (SH) solutions with three different molarities (8M, 10M, and 14M) were used. Steel fibers were added to the geopolymer concrete mix in varying proportions up to 3%, by volume. Constant binder, activator solution, and aggregate contents were adopted for all 13 mixes. Samples were cast and cured at ambient conditions for measuring the rheological and mechanical properties, including slump, modulus of elasticity, compressive strength, tensile splitting strength, and flexural strength. Experimental test results show that geopolymers made with higher molarity of SH were less workable but had improved mechanical performance. The effect of adding steel fibers on the mechanical performance was more apparent at an early age and in weaker geopolymer concretes. Additionally, scanning electron microscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy highlighted the co-existence of calcium aluminosilicate hydrate and sodium aluminosilicate hydrate gels.