To decrease the water absorption capacity of hemp fibers and increase the final mechanical properties of short hemp fiber-reinforced fly ash-based geopolymer mortars after exposure to 10 wet/dry cycles, two distinct fiber treatments were employed, i.e., fiber hornification (with 5, 10, or 15 cycles) and NaOH treatment (with 3%, 6%, or 9% of NaOH). Both treatments proved to increase the final mechanical properties of non-treated hemp fiber-reinforced geopolymers after exposure to 10 wet/dry cycles. After wet/dry cycles, when the geopolymers were reinforced with the hornificated fibers, the compressive strength increased up to 80%, flexural strength up to 22%, and energy absorption capacity up to 18% in comparison to the geopolymer reinforced with non-treated fibers. Similarly, the geopolymers reinforced with NaOH treated fibers, after being exposed to wet/dry cycles, resulted in a 49%, 25%, and 19% increase in compressive- and flexural strengths, and energy absorption capacity, respectively, compared to its counterpart reinforced with non-treated fibers.