With notable increases in older adult populations, as well as with the associated cognitive impairments that can accompany aging, there is significant importance in identifying strategies to promote cognitive health. The current study explored the implementation of a positive reminiscence program (REMPOS), a non-pharmacological cognitive therapy that has been previously structured, defined, and tested in a Spanish sample. We sought to improve the quality of life of institutionalized older adults with healthy aging, mild cognitive impairment, and Alzheimer’s disease by utilizing this protocol in these samples. A randomized design with a pre-post measure was conducted over a three-month period. Two types of interventions were used: the experimental groups received REMPOS, and the control groups underwent their regular daily institutional programming with cognitive stimulation techniques. After the intervention, the three experimental groups showed higher cognitive functioning, decreased depressive symptomatology (except for the MCI group) and higher evocation of specific positive memories (except for the MCI group). This study supports the effectiveness of REMPOS and reminiscence therapy, with regard to both cognitive and mood factors in cognitively impaired older adults.