This study investigates the quasi-static indentation and post-fire behavior of foam-core glass fiber-reinforced polymer (GFRP) sandwich composites modified with varying weight fractions (0, 1, and 2 wt.%) of magnesium hydroxide as a flame-retardant filler. Experimental results demonstrated that the incorporation of magnesium hydroxide significantly enhanced both mechanical performance and fire resistance. The peak load increased from 0.383 kN to 0.433 kN by including 2 wt.% additive prior to fire exposure, and further to 0.570 kN following fire exposure. Energy absorption exhibited a similar trend, and rising from 4.25 J to 4.994 J by including 2 wt.% magnesium hydroxide in the composite. Moreover, the magnesium hydroxide included specimen showed an energy absorption of 5.662 J after fire exposure. These findings confirm that incorporating magnesium hydroxide provides optimal structural integrity and thermal protection, positioning it as a promising filler for GFRP sandwich composites intended for fire-critical structural applications.