Although waste glass aggregate has been widely investigated as a substitute for natural fine aggregate in concrete, the microstructural mechanisms governing its effects on pore evolution, interfacial transition zone (ITZ) characteristics, and mechanical response remain insufficiently clarified. This study investigated concrete containing 0%−100% waste glass fine aggregate (WGFA) at different curing ages using mechanical tests, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The results show that WGFA incorporation changes the pore structure evolution from typical capillary pore refinement to a coupled process involving capillary pore reduction and partial filling of smaller pores. Under high WGFA replacement conditions, porosity decreases while the mean pore diameter increases, indicating a redistribution rather than simple refinement of pore structure. Meanwhile, the smooth and low-water-absorption surface of WGFA progressively weakens the ITZ and promotes interface microcracking. These microstructural changes lead to different mechanical responses: compressive strength is mainly governed by matrix densification, whereas flexural strength is more sensitive to ITZ deterioration. WGFA concrete exhibits a transition from matrix-densification-dominated behavior at low replacement levels to ITZ-cracking-controlled behavior at high replacement levels, and the recommended WGFA replacement ratio should be controlled within 20%.