The susceptibility of high-strength cementitious composites to explosive spalling under elevated temperatures necessitates the development of sustainable, fire-resistant alternatives for structural applications. This study comparatively evaluates the thermo-mechanical performance and spalling resistance of high-strength alkali-activated slag mortar (HSAAM) and ordinary Portland cement-based mortar (HSCM) under rapid fire scenarios. HSAAM was synthesized using granulated blast-furnace slag (GGBFS), while HSCM incorporated silica fume (SF) to achieve comparable compressive strength. Specimens were exposed to short-term elevated temperatures (200–600 °C) at 10 °C/min, with dwell times of 10–30 min, followed by furnace cooling or water quenching. Residual mechanical properties (compressive strength, tensile strength, impact resistance), thermal insulation, mass loss, and microstructural evolution were systematically analyzed. Results revealed that HSAAM exhibited complete spalling resistance up to 600 °C, whereas HSCM suffered partial spalling at 400 °C and catastrophic failure under water cooling. After 10 min at 400 °C, HSAAM retained 66.8% compressive strength (52 MPa) and 82% tensile strength (1.66 MPa), while HSCM retained 102% compressive strength (77.5 MPa) but experienced 10% specimen failure. HSAAM demonstrated superior thermal insulation, with core temperatures 44% lower than HSCM at 400 °C. Microstructural analysis via SEM/EDS identified a nano-porous matrix in HSAAM, facilitating vapor release and mitigating internal pressure. These findings position alkali-activated slag mortars as a robust, fire-resilient alternative to conventional cementitious systems in high-temperature environments.