Considering the difficulty in finding a reversible cure for dementia, research and clinical focus have shifted towards modifiable risk factors for cognitive decline, with sleep disturbances gaining attention especially in older adults with mild cognitive impairment (MCI). Given the significant impact of sleep disturbances on cognitive decline in MCI, this study explores the relationship between psychological factors, including depression, anxiety, and stress, and insomnia among older adults with MCI. 145 Greek older adults were recruited from an outpatient memory clinic in Athens. The sample met the following inclusion requirements: Petersen’s criteria of MCI diagnosis, absence of a psychiatric history or chronic organic disease, and Greek mother language. All participants completed the Athens Insomnia Scale (AIS-8) and the Depression, Anxiety, Stress Scale (DASS-21). The regression analysis indicated that clinical symptoms of depression is a significant predictor of insomnia severity, with higher levels of depression being associated with more severe insomnia symptoms. However, anxiety and stress did not significantly predict insomnia severity in this study. Gender and MCI subtype did not significantly affect the degree of insomnia. Findings emphasize the importance of treating depression as a potential modifiable risk factor for insomnia, which in turn can further impair cognitive function in individuals with MCI. Further research is needed to explore additional factors contributing to cognitive impairment and to develop targeted interventions.