Background Conduct problems (CP) and attention deficit hyperactivity disorder (ADHD) are often comorbid and have each been linked to ‘unhealthy diet’. Early‐life diet also associates with DNA methylation of the insulin‐like growth factor 2 gene (IGF2), involved in fetal and neural development. We investigated the degree to which prenatal high‐fat and ‐sugar diet might relate to ADHD symptoms via IGF2 DNA methylation for early‐onset persistent (EOP) versus low CP youth. Methods Participants were 164 youth with EOP (n = 83) versus low (n = 81) CP drawn from the Avon Longitudinal Study of Parents and Children. We assessed if the interrelationships between high‐fat and ‐sugar diet (prenatal, postnatal), IGF2 methylation (birth and age 7, collected from blood), and ADHD symptoms (age 7–13) differed for EOP versus low CP youth. Results Prenatal ‘unhealthy diet’ was positively associated with IGF2 methylation at birth for both the EOP and low CP youth. For EOP only: (a) higher IGF2 methylation predicted ADHD symptoms; and (b) prenatal ‘unhealthy diet’ was associated with higher ADHD symptoms indirectly via higher IGF2 methylation. Conclusions Preventing ‘unhealthy diet’ in pregnancy might reduce the risk of ADHD symptoms in EOP youth via lower offspring IGF2 methylation.