The influence of prolonged high-temperature aging on the microstructural evolution and mechanical properties of GH4169 nickel-based superalloy was investigated. The results demonstrate that the standard heat-treated alloy primarily consists of a γ-phase matrix with trace amounts of short rod-shaped δ-phase (0.139% volume fraction, 0.32 μm in size) distributed along grain boundaries, achieving optimal mechanical properties: yield strength 1330 MPa, ultimate tensile strength 1385 MPa, and hardness 497HV, albeit with limited ductility 19.75% reduction of area. During aging at 900 °C for 500-1134 h, the metastable γ'' phase progressively transforms into δ-phase (Ni₃Nb), accompanied by a substantial increase in δ-phase content (14.2% at 500 h and 15.1% at 1134 h) and coarsening from 2.65 μm to 2.86 μm. The δ-phase evolves distinct microstructural characteristics: intergranular short-rod fragmentation and intragranular oriented needle-like precipitation. Significant aging effects are observed, manifesting as a 57% hardness reduction, 34% yield strength decline, 19% decrease in impact toughness, and an 80% ductility improvement. Mechanistic analysis reveals that initial grain-boundary δ-phase enhances the strength via grain refinement through Zener pinning. However, prolonged aging induces δ-phase coarsening at grain boundaries, which consumes γ'' strengthening precipitates and weakens the matrix. Concurrently, high-temperature aging promotes direct intragranular δ-phase precipitation, deteriorating the precipitation strengthening efficacy. Fractography transitions from equiaxed dimples (ductile fracture) in the standard condition to a ductile-brittle mixed mode featuring willow-leaf-shaped dimples and cleavage planes in the aged alloys, with δ-phase interfaces acting as preferential pathways for crack propagation. These findings elucidate the critical role of δ-phase evolution in governing the strength-ductility trade-off during high-temperature aging.