Reinforcement corrosion can alter the seismic response of reinforced concrete bridges throughout their service life. Previous studies have either investigated the effects of corrosion on strength, drift, and seismic fragility or applied cumulative damage indices to bridges without considering corrosion scenarios. Consequently, limited attention has been given to how corrosion progression modifies the relationship between maximum deformation and the effects of inelastic loading cycles, and whether a fixed drift threshold corresponds to the same level of cumulative damage at different bridge ages. To address this issue, a three-dimensional nonlinear model of a reinforced concrete bridge was developed in OpenSees for three conditions: uncorroded, 25-year corrosion, and 50-year corrosion. Incremental dynamic analyses were conducted using 22 far-field ground-motion records. Maximum drift, aggregate Park–Ang damage index, and seismic fragility curves were obtained, and the relationship between the two response measures was evaluated descriptively and on a record-by-record basis. The results showed that corrosion shifted the fragility curves toward lower intensity levels and reduced the median capacity of the 50-year model approximately 10% to 16%, depending on the damage state. A power-law fit of the drift–Park–Ang relationship across the three scenarios yielded values ranging from 0.93 to 0.94, and indicated that corrosion shifted the level of the relationship between the two measures. Moreover, the Park–Ang values corresponding to fixed drift thresholds in the 50-year model were approximately 20% to 23% lower than those of the uncorroded model. These findings demonstrate that a fixed drift threshold does not represent the same level of cumulative damage at different bridge ages, and the quantitative relationship between drift and the calculated Park–Ang index does not remain constant as the corrosion condition changes. Therefore, performance assessment of corroded bridges should account not only for changes in seismic fragility curves but also for corrosion-dependent variations in the drift–Park–Ang relationship.