Background: The Mediterranean diet (MD) is a beneficial dietary pattern with strong antioxidant and anti-inflammatory properties that can promote mental and physical human health. This study aims to assess the impact of MD adherence on health-related quality of life, physical activity levels, and sleep quality in a representative Greek elderly population. Methods: This is a cross-sectional study. A total of 3254 persons ≥65 years from 14 different Greek regions, urban, rural and islands participated in this study, of which 48.4% were female and 51.6% were male. Health-Related Quality of Life (HRQOL) was evaluated utilizing a short form healthy survey, physical activity was determined by the International Physical Activity Questionnaire (IPAQ), sleep quality was assessed utilizing the Pittsburgh Sleep Quality Index (PSQI) and MD adherence was assessed via the Mediterranean Diet Score (MedDietScore). Results: Moderate adherence to the MD and an increased prevalence of poor quality of life, low physical activity levels and inadequate sleep quality among the elderly population were recorded. High MD adherence was independently associated with better quality of life (OR: 2.31, 95% CI: 2.06–2.68,p= 0.0008), higher physical activity (OR: 1.89, 95% CI: 1.47–2.35,p= 0.0141) and adequate sleep quality (OR: 2.11, 95%: 1.79–2.44,p= 0.0018), female sex (OR: 1.36, 95% CI: 1.02–1.68,p= 0.0032) and living with others (OR: 1.24, 95% CI: 0.81–1.76,p= 0.0375), after adjustment for potential confounding factors. In unadjusted analysis, participants’ age (p< 0.0001), anthropometric characteristics (p< 0.005), educational (p= 0.0026) and financial status (p= 0.0005) and smoking habits (p= 0.0031) were also identified as indicators of MD adherence; however, their impact on MD adherence was considerably attenuated after adjusting for confounding factors (p> 0.05). Conclusion: High MD adherence was correlated with favorable quality of life, higher levels of physical activity, and a more adequate sleep quality score. Strategies and public health policies that facilitate MD adherence and physical activity in older adults may improve sleep and quality of life, impacting overall wellbeing in this age group.