Background Attention deficit hyperactivity disorder is generally widely under-recognised and undertreated in adults. Real world evidence is needed to characterise healthcare resource utilisation and economic burden of adult attention deficit hyperactivity disorder in England. Methods This retrospective cohort study used primary care medical records (Clinical Practice Research Datalink – Aurum) linked to secondary care data (hospital episode statistics). Patients were indexed on their first-ever record of an attention deficit hyperactivity disorder diagnosis, made during adulthood, between 1 January 2016 and 30 June 2018. Healthcare resource utilisation and associated costs were compared 18-months before and after diagnosis. Results 2236 patients were included; mood disorders and anxiety were recorded in 47.1% and 46.2%, respectively. In 2153 patients with available data for both periods, mean total healthcare resource utilisation cost per patient was £408.6 (95% CI £264.4–£552.8) higher in the post- versus pre-index period (p < 0.001). The mean prescriptions cost per patient doubled (£322.5 versus £614.5) and represented the largest proportional cost increase post-diagnosis compared to other healthcare resource utilisation costs measured. Discussion An attention deficit hyperactivity disorder diagnosis in adulthood ensures patients receive appropriate treatment for attention deficit hyperactivity disorder and comorbid anxiety and depression. Total healthcare resource utilisation costs appeared to increase post attention deficit hyperactivity disorder diagnosis; however, further studies are required to confirm this. This cost increase should be set in the context of multiple societal benefits likely to accrue from the effective management of adult attention deficit hyperactivity disorder.