To investigate the elevated-temperature performance of Ultra-High-Performance Concrete (UHPC) and Ultra-High-Performance Geopolymer Concrete (UHPGC), a systematic comparative study was conducted at 800 °C. This study examined the effects of the steel fibre geometry (micro and hooked-end) and dosage (1.5% and 2.0%) on mass loss, crack propagation, residual compressive, flexural, and tensile strengths, and chemical evolution following a 24 h pre-drying protocol to mitigate explosive spalling. The results demonstrate that UHPGC exhibits superior thermal stability and residual mechanical performance compared with UHPC after high-temperature exposure. Among all mixtures, the UHPGC mixture reinforced with 2% micro steel fibres (UHPGC-M2) achieved the highest residual compressive strength (30 ± 0.4 MPa, corresponding to 25% strength retention compared with 21% for the equivalent UHPC mixture), the lowest post-exposure crack width (0.08 mm), and the highest tensile strength retention (17.9%). Micro steel fibres were more effective in controlling crack propagation and preserving peak load capacity, whereas hooked-end fibres contributed more significantly to post-peak ductility. Chemical analysis revealed substantial chemical changes in both systems after exposure to 800 °C. However, UHPGC exhibited lower mass loss (4.8%) and greater residual performance. These findings establish micro steel fibre-reinforced UHPGC as a sustainable and high-performance material for fire-resistant structural applications.