Objective: The aim of the study was to compare the development of working memory and preliminary stages of attentional processing in individuals with ADHD over a 23- to 25-year period. Method: Individuals with ADHD ( n = 19) and healthy controls ( n = 26) were followed up after 13 years (T2) and 23 to 25 years (T3) after initial assessment (T1). They were reassessed with diagnostic measures and the Backward masking task (pre-attention) and the Digit span distractibility test with and without distraction conditions (working memory). Results: The ADHD group performed below the healthy controls on all time points on the Digit span distractibility test. On the distractibility condition, we found a selective decline in performance from T2 to T3 for the ADHD group. Conclusion: The results highlight that ADHD individuals continue to display working memory deficits, also in adulthood, thus creating an imperative for cognitive rehabilitation techniques to help address attention difficulties.