Compassion Mind Training (CMT) is a therapeutic approach to guide highly self-critical individuals to generate compassion. The goal was to probe the efficacy of a short-term, online version of the CMT on self-compassion and self-criticism in a non-clinical population. We conducted a randomized controlled trial with pre-, post-measurements and two-month follow-up. Out of 144 randomly allocated participants 26 and 20 of those allocated to the intervention and control groups, respectively, completed the follow-up measures. The intervention group was instructed through email to practice a different CMT exercise every day for 13 consecutive days. There was a significant effect of the intervention on self-criticism, especially Hated-self and the Self-uncompassionate responding. The CMT group reported a reduction in negative thoughts and feelings with effects present at the two-month follow-up. There was no significant effect of the intervention on self-reassurance and self-compassion. Self-criticism is amenable to change following a short-term online intervention of CMT delivered to a non-clinical population with effects lasting at least two months. These findings are promising and suggest that interventions designed to reduce self-criticism can be provided to broader populations without direct involvement of mental health professionals.