There is a high prevalence of insomnia in older adults, which has negative consequences for their well-being and quality of life. The recommendation for first-line treatments is to administer non-pharmacological interventions. The objective of this research was to verify the effectiveness of Mindfulness-Based Cognitive Therapy in older adults with subclinical and moderate insomnia, studying its impact on sleep quality. One hundred and six older adults participated and were assigned to the subclinical insomnia (n = 50) or moderate insomnia (n = 56) groups; subsequently, they were randomly assigned to the control and intervention groups. Subjects were evaluated at two times with the Insomnia Severity Index and the Pittsburgh Sleep Quality Index. Significant results were obtained on both scales, with a reduction in insomnia symptoms in the subclinical and moderate intervention groups. The administration of a treatment that combines mindfulness and cognitive therapy is effective for treating insomnia in older adults.