The risk of explosive spalling in high-strength cement-based materials during fire exposure poses a significant threat to structural integrity. To help mitigate this issue, this study explores the use of expanded polystyrene (EPS) beads as both a lightweight filler and a potential spalling-reduction agent in lightweight geopolymer and conventional cementitious mortars. Two EPS-containing mortars were developed: a lightweight alkali-activated slag (LWAS) mortar and a conventional lightweight Portland cement (LWPC) mortar, both incorporating EPS beads as a 50% volumetric replacement for sand. Specimens from both mortars were subjected to elevated temperatures of 200 °C, 400 °C, and 600 °C at a heating rate of 10 °C/min to simulate a rapid-fire scenario. Following thermal exposure, two cooling regimes were employed: gradual cooling within the furnace and rapid cooling by water immersion. Mechanical performance was evaluated through compressive, splitting tensile, and impact tests at room and elevated temperatures. Microstructural analysis was also conducted to examine internal changes and heat-induced damage. The results indicated that LWAS showed remarkable resistance to spalling, remaining intact up to 600 °C due to its nanoporous geopolymer structure, which allowed controlled steam release, while LWPC failed explosively at 550 °C despite EPS pores. At 400 °C, EPS beads enhanced thermal insulation in LWAS, lowering internal temperature by over 100 °C, but increased porosity led to faster strength loss. Both mortars gained strength at 200 °C from continued curing, yet LWAS retained strength better at high temperatures than LWPC. Microscopy revealed that EPS created beneficial fine cracks in the slag matrix but harmful voids in cement. Overall, LWAS composites offer excellent spalling resistance for fire-prone environments, though reinforcement is recommended to mitigate strength loss.