The mediators of real-world executive functions in autism during the transition into adulthood are mainly unknown. This study aimed to identify the mediators for the behavioral and cognitive domains of real-world executive functions in late adolescent and young adult autistic populations. We followed up 289 autistic children (aged 11.6 ± 3.8, male 82.2%) and 203 non-autistic controls (10.7 ± 2.9, 66.5%) to their ages of 16.9 ± 4.7 and 15.8 ± 3.9, respectively. The Behavior Rating Inventory of Executive Function, comprising two global scales (Behavioral Regulation Index and the Metacognitive Index), was used to assess real-world executive functions at Time 2. Mediators of behavior regulation and metacognition were explored among a wide range of clinical correlates and parental bonding. We found that the autistic group had lower executive functions than the non-autistic group with lower behavior regulation and metacognition subscores. Several childhood factors were significant mediators. Multiple mediation analyses revealed that childhood inattention remained a significant mediator for both behavior regulation and metacognition in late adolescence, and peer problems were the specific mediators of metacognition. Our findings suggest several childhood factors may mediate real-world executive functions during late adolescence. Treating common mediators such as inattention may improve real-world executive functions in autistic individuals during adulthood.