Amnestic mild cognitive impairment (aMCI), an Alzheimer's disease prodrome, is characterized by cognitive and psychological symptoms, the latter aggravating prognosis. A mindfulness-based intervention (MBI) represents a promising non-pharmacological framework for Alzheimer's disease prevention. The Monitoring + Acceptance Theory (MAT) postulates that MBI improves cognition through monitoring, and psychological well-being, through acceptance. This single-blind preliminary randomized-controlled study investigated the effects of a MBI on anxio-depressive symptoms, quality of life, and memory, compared to a psychoeducation-based intervention in older adults with aMCI. The contribution of MAT components and of ruminations' reduction to intervention efficacy were examined. Participants assigned to both conditions experienced similar benefits regarding anxio-depressive symptoms and aging-related quality of life. General quality of life and memory remained unchanged. A partial support of the MAT and of ruminations reduction to the MBI's efficacy was found. The findings provide new insights on the effects and mechanisms of a MBI on aMCI symptoms.