Aim: This study examined the change over 20 months in 178 participants with frequent episodic migraine under adequate treatment as usual, who had completed online behavioural training (oBT) in migraine self-management either directly (group 1, n = 120) or after 10 months of watchful waiting (group 2, n = 58). Methods: Participants completed questionnaires and an online headache diary and migraine monitor following the International Classification of Headache Disorders at T0 (baseline), T1 (post-training), T2 (6-month follow-up; extended baseline in group 2), T3 (post-training, group 2 only) and T4 (group 1: 16-month follow-up; group 2: 6-month follow-up). Statistical analyses were conducted on the observed data without imputation of missing observations. Results: Both groups were highly comparable. The data over time revealed benefits in response to oBT, with significant between-group differences in the change achieved in the training episodes T2-T0 (group 1) and T4-T2 (group 2). Improved attack frequency (M = -23%) was higher in participants with more (i.e. 4-6) attacks per month at baseline, and the effects of oBT were durable over 16 months of follow-up. Conclusions: oBT is beneficial in frequent episodic migraine, which deserves special efforts in care. Self-management variants such as oBT reach patients easily and supplement these efforts with durable results.