A laboratory investigation using typical Florida dense-graded and open-graded mixtures was conducted to evaluate the effects of a few promising modifiers on the long-term aging characteristics of asphalt mixtures. Six types of modifiers including gilsonite, carbon black, fine ground tire rubber (GTR-80), styrene-butylene-rubber (SBR), ethylene vinyl acetate (EVA) and styrene-ethylene-butylene-styrene (SEBS) were used in this study. These modifiers were blended with AC-30, AC-20, and AC-5 base asphalts to produce a total of ten modified asphalts. These ten modified asphalt blends and the unmodified AC-30 were used to make Florida type S-1 structural course and type FC-2 friction course asphalt mixtures. These mixtures were compacted into Marshall specimens for age hardening and subsequent testing. A total of 416 compacted modified and unmodified asphalt mixture specimens were subjected to a forced-draft oven aging process for 90 days at 60 °C (140 °F), and natural sunlight for 6 months to simulate the long-term aging effects on these mixtures. These aged and unaged asphalt mixtures were evaluated by (1) resilient modulus tests at 5 and 25 °C (41 and 77 °F) and (2) indirect tensile tests at 5 and 25 °C (41 and 77 °F). The effects of cycle frequency used in the resilient modulus tests were also assessed. The test results indicate that the modified asphalt mixtures appear to have more pronounced delayed elastic behavior as compared with that of the unmodified asphalt mixtures. The fracture energy, which is characterized by means of the area of stress-strain curve, indicated that the modified binders using a base asphalt of AC-30 tend to be too brittle as they harden further with time. However, modified binders using a base asphalt of AC-5 appear to be softer than that of conventional AC-30 under typical Florida conditions.