This study assesses the effects of three binder systems—Portland cement CEM I 52.5 N, CEM I 52.5 N with 8 % silica fume, and slag-based CEM III/A 52.5 L—on the mechanical performance and residual characteristics of plain and steel fibre-reinforced mortars subjected to high temperatures. Mortars were subjected to regulated heating and cooling cycles at a rate of 5 °C/min to achieve peak temperatures of 400, 500, and 800 °C. Compressive and tensile-bending strengths, post-peak behaviour, modulus of elasticity, fracture energy, residual strength intensity factors, and toughness indices were evaluated. The findings indicate that steel fibres significantly enhance the residual performance of all binder systems, with CEM I-based mortars exhibiting enhanced strength retention and stiffness at all temperatures. Silica fume-modified CEM I mortars (SFRMS) demonstrated enhanced tensile-bending strength and ductility, attributable to a more compact microstructure and superior fibre–matrix adhesion. CEM III/A mortars exhibited increased residual cracking energy at intermediate temperatures (400–500 °C), highlighting the transient benefits of slag in mitigating crack propagation. However, at temperatures exceeding 500 °C, all mortars experienced significant degradation, with SEM analysis revealing considerable microstructural damage and fibre oxidation at 800 °C. This study highlights the efficacy of combining steel fibres with extra cementitious materials to reduce thermal damage, enhance fracture resistance, and prolong the post-peak load-bearing capacity of mortars. The results underscore the efficacy of optimised steel fibre-reinforced mortars in the repair and protection of concrete structures subjected to intense thermal conditions, facilitating mechanical recovery after fire incidents to improve structural safety.