Objective: Children with ADHD often experience functional impairments across various aspects of daily life. This study addresses the dearth of longitudinal research on functional impairment trajectories from preschool to school age in children with symptoms of ADHD and comorbid disorders. Methods: We investigated the extent to which functional impairments were associated with ADHD symptoms, along with behavioral and anxiety symptoms, from age 3.5 to 8 years. Utilizing parent- and teacher-reported data, we analyzed associations between global impairment, and dimension scores (e.g., family; child quality of life (QoL); learning; play/leisure activities; and friends) and symptoms at ages 3.5 and 8 years (n = 783). Results: The mean parent global impairment score increased from 0.31 (standard deviation (SD) = 0.40) to 0.83 (SD = 0.63) from 3.5 to 8 years, while the teacher impairment scores slightly decreased. Specific parent impairment dimension scores, particularly QoL, learning, and friends, significantly increased. Preschool ADHD and comorbid behavioral symptoms reported by parents weakly predicted impairment at 8 years. By age 8 years, impairment and symptoms exhibited moderate to strong correlations for all impairment dimensions. Parents reported greater child impairment during school age across settings compared to preschool, while teachers' impairment profiles remained consistent across ages. Conclusions: These findings suggest that parents perceive impairment as more pronounced at age 8 years and more strongly associated with symptoms of both ADHD and comorbid disorders than at age 3.5 years. Notably, for teachers, a robust correlation between inattention symptoms and learning impairment was observed, with substantially higher impairment scores reported for boys compared to girls.