The aim of this study was to investigate the influence of the fibres hornification (10 and 15 wetting/drying cycles) on the mechanical properties of hemp fibre reinforced fly-ash based geopolymer mortars. The geopolymer mortars were reinforced with 10mm long hemp (Cannabis sativa L) fibres in the dosage of 1.0 vol%. Their compressive and flexural strengths, as well as energy absorption capacity were measured at the age of 28 days. It was shown that the hornification of the fibres led to the fibres’ separation (fibrillation) and fibres’ cleaner surfaces. This probably led to fibres’ better distribution through the matrix, which consequently led to less porous and denser mortars. All this influenced the increase in all tested mechanical properties of the fibre reinforced geopolymer mortars, after the fibres hornification. The difference of the hornification number (either 10 or 15) showed no significant influence on the fibre reinforced mortar’s mechanical properties. Since this method of fibres treatment is easy to apply and financially and environmentally justified, it is highly recommended to be used for the future natural fibre reinforced mortars’ investigation.