Longitudinal analyses have revealed informative patterns about health, mental health, adaptive skills, autism symptoms, and social skills during the transition to adulthood for autistic individuals. This study expands on these findings by investigating the trajectories of positive affect from age 15 to 30 years using a heterogeneous cohort (n = 253) of individuals first referred for autism at an early age. Group-based trajectory modeling was used to identify commonalities in trajectories using both caregiver-reported and self-reported positive affect. We analyzed differences between these trajectory groups on demographic and behavioral measures, as well as indices of adult functioning characterized for either higher and lower cognitive abilities. Caregiver-reported values revealed four different patterns of stability and change, and self-reported values revealed three distinct patterns of stability and change with variable intercepts. These trajectory groups differed by autism severity, intelligence quotient, daily living skills, and different indices of adult outcomes, including social relationships, work, and activity engagement. There were some differences in trajectory shape by reporter, with agreement between caregiver-report and self-report being limited after age 23 years. The results of this study have implications for how we measure subjective indices of experience across the spectrum of cognitive abilities present in autism.