Introduction: Major depressive disorder (MDD) is common, and recurrence rates are high. Preventive Cognitive Therapy (PCT), has been shown to prolong time to recurrence and reduce risk of recurrence(s) over 2–10 years in patients with recurrent depression. Objective: The aim of the study was to examine the effectiveness of PCT over 20 years on time to first recurrence, cumulative proportion of first recurrences, percentage of depression-free time, mean severity of recurrences, and the number of recurrences within a patient. Methods: Adults remitted from recurrent MDD were randomized to PCT or Treatment As Usual (TAU). Clinical outcomes were assessed using the SCID over 20 years. We used Cox regression analyses, Kaplan-Meier analyses, ANOVA, and negative binomial regression and tested for interaction with the number of previous episodes. Results: There was a significant interaction effect for number of previous episodes with treatment condition on time to first recurrence (Wald[1, n = 172] = 8.840, p = 0.003). For participants with more than 3 previous episodes, the mean time to recurrence was 4.8 years for PCT versus 1.6 years for TAU; the cumulative proportion of first recurrences was 87.5% for PCT and 100% for TAU. For participants with more than 3 previous episodes, exploratory analyses suggest that PCT had 53% less recurrences and percentage of depression-free time was significantly higher compared to TAU. There were no significant effects on mean severity. Conclusions: Up to 20 years, for MDD patients with more than 3 previous episodes, those who received PCT had significantly longer time to a first recurrence and lower recurrence risk and may have less recurrences and more depression-free time compared to TAU. This suggests long term protective effects of PCT up to 20-years.