Prolonged high-concentration CO₂ mineralization may enhance early-age strength but cause a late-age strength decline in cementitious backfill materials. This study investigated how ground glass fiber (GGF) dosage (0%, 1%, and 2%) affects fly ash–cement backfill materials under conventional hydration and prolonged CO₂ mineralization curing. Compressive strength was measured at 3, 7, and 28 d, together with X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, mercury intrusion porosimetry, and scanning electron microscopy with energy-dispersive spectroscopy. Under CO₂ mineralization, the 0% GGF mixture decreased from 22.42 MPa at 7 d to 14.32 MPa at 28 d, indicating a 36.1% strength decline. Increasing GGF dosage to 1% increased the 28 d strength to 21.07 MPa and reduced the 7–28 d strength decline to 22.2%, whereas 2% GGF provided no further improvement. GGF also increased strength under conventional hydration, indicating a general strengthening effect together with improved late-age strength retention under CO₂ curing. TGA showed increased carbonate-related mass loss after mineralization, but 1% GGF did not further increase it, while MIP showed lower total porosity but larger median pore size. These results indicate that the strength benefit of 1% GGF was not governed by greater carbonate formation or overall pore refinement and may involve local particle–matrix interactions.