Background: This study examined symptoms and lifetime course of Attention Deficit Hyperactivity Disorder (ADHD) in adults with borderline and mild Intellectual Disability (ID). Method: A total of 48 adults with ID and ADHD were compared with 221 adults with ADHD without ID using the informant Barkley scale for childhood and adulthood symptoms. Results: The ADHD/ID group presented with greater severity of (adult and childhood) symptoms compared with the non-ID group. For the ADHD/non-ID group, most symptoms improved significantly from childhood to adulthood, whereas only two symptoms changed significantly for the ID group. Principal component analysis revealed scattered loading of different items into five components for the ADHD/ID group that were not consistent with the classic clusters of inattentive, hyperactive and impulsive symptoms. A negative correlation was found between severity of symptoms and IQ. Conclusions: ADHD in adults with ID may have a more severe presentation and an uneven and less favourable pattern of improvement across the lifespan in comparison with adults without ID.