The need for physical distancing due to COVID-19 mitigation efforts forced prolonged social isolation, which may affect sleep and lead to mental health problems. Previous research has shown that young adults are particularly vulnerable to psychological stress caused by social isolation, the negative psychological impact of the pandemic, and greater frequency and severity of sleep problems. Therefore, the main goal of the present study was to examine whether insomnia could constitute a mediation mechanism that explains the relationship between social isolation experienced during the COVID-19 pandemic and mental health outcomes (depression and anxiety) reported up to 1.5 years later. The study was conducted among young (M±SD; 24.08±3.75) men (N = 1025) in Poland. Data were collected by means of self-report questionnaires, includingThe Social Isolation Index,The Athens Insomnia Scale,The State-Trait Anxiety Inventory (STAI-S)andBeck’s Depression Inventory (BDI-II). The results show that insomnia mediates the relationships between social isolation and both anxiety and depression. The current findings emphasize the role of insomnia in the relationships between social isolation experienced during COVID-19 and negative emotional states. From a clinical perspective, the results suggest that implementing therapeutic components that address social isolation in insomnia treatment programs may prevent the development of depression and anxiety symptoms among young men.