Background: Children undergoing open heart surgery are at risk of neurological injury. A cohort of 35 patients, who had undergone cardiac surgery during infancy, had a significant reduction in Bayley Scale of Infant Development scores at a 12-month assessment. This cohort has now reached an appropriate age to reassess developmental progress. Methods: Detailed psychometric testing was conducted on 20 children from the original cohort using the Weschler Preschool and Primary Scale of Intelligence, the Wide Range Assessment of Memory and Learning, and the Wechsler Individual Achievement Test. Parents completed the Connor’s Rating Scale, the Behaviour Rating Scale of Executive Functioning, and the Child Behaviour Checklist. Results: The mean age of the cohort at assessment was 6.6 (standard deviation 0.4) years. Mean scores on all tests of intelligence, memory, academic achievement, and executive function fell within the average range. Of the children, 20–35% were found to have significant difficulties across these areas. Mean scores in the areas of social, emotional, behavioural, and psychological functioning also fell within the average range. Of the children studied, 35% had clinically significant problems in these areas. There was only a weak association between the 12-month scores and the Full-Scale Intelligence Quotient at 6 years. Conclusion: Detailed psychometric testing of these children suggests that they generally function in the average range; however, a significant proportion falls below age expectations in all the areas assessed. This highlights the importance of long-term follow-up with routine developmental screening to allow identification of a subgroup that may benefit from early educational and behavioural intervention.