Although externally bonded fiber-reinforced polymer (FRP) composites are commonly employed to strengthen reinforced concrete members, the durability of these rehabilitation measures is not well understood. This work includes the examination of FRP-strengthened girders after 12 years of service exposed to brackish water, including assessments of remaining structural strength and investigations of the FRP. Structural load tests of the rehabilitated girders—compared to companion, unstrengthened girders—demonstrate that FRP strengthening maintains influence on the girder strength and failure mode at ultimate strength. The combination of glass fiber–reinforced polymer (GFRP) and carbon fiber–reinforced polymer (CFRP) reinforcement provided a modest improvement in bridge girder strength (12% beyond residual strength) while limiting ductility. Inspection of the FRP microstructure, mechanical property tests, and bond pull-off tests, however, suggest that the FRP’s contribution to structural strength may be greatly influenced by construction techniques and exposure conditions. Strength was calculated using the strain limits from relevant guidelines and was conservatively estimated for the design scenario herein evaluated.