Background: Mindfulness-based cognitive therapy (MBCT) is a potential treatment for chronic insomnia. We evaluated the efficacy of MBCT for insomnia (MBCT-I) by comparing it with a sleep psycho-education with exercise control (PEEC) group. Methods: Adults with chronic primary insomnia (n = 216) were randomly allocated to the MBCT-I or PEEC group. The MBCT-I included mindfulness and psycho-education with cognitive and behavioural components under cognitive behavioural therapy for insomnia. PEEC included psycho-education of sleep hygiene and stimulus control, and exercises. Any change in insomnia severity was measured by the Insomnia Severity Index (ISI). Secondary outcomes included sleep parameters measured by a sleep diary, health service utilisation, absence from work and mindfulness measured by the Five Facet Mindfulness Questionnaire. Results: The ISI score significantly decreased in the MBCT-I group compared with the PEEC group at 2 months (i.e., post-intervention) (p = 0.023, effect size [95% CI] -0.360 [-0.675, -0.046]) but not at 5 or 8 months. Treatment response rates and remission rates based on the ISI cut-off scores were not significantly different between groups. Wake time after sleep onset (WASO) was less in the MBCT-I group at 2 and 5 months. At 8 months, both groups showed a reduced ISI score, sleep onset latency and WASO, and increased sleep efficiency and total sleep time; however, no group differences were seen. Other outcome measures did not significantly improve in either group. Conclusions: Long-term benefits were not seen in MBCT-I when compared with PEEC, although short-term benefits were seen.