BACKGROUND: Robust public health and health system response to the increasing burden of multimorbidity worldwide requires detailed epidemiological examination of its key sociodemographic and geographic determinants. We investigated the role of gender, age and socioeconomic and geographic factors on multimorbidity (i.e., having two or more conditions) in the adult population in France and examined implications for surveillance and prevention. METHODS: We used data from two large nationwide representative surveys with cross-sectional and longitudinal health and socio-demographic indicators, conducted in France between 2008 and 2014. Morbidity counts and frequent dyads/triads of conditions independently impacting mortality, activity limitations, and perceived health were investigated with regard to differences in gender, age, socioeconomic (education, occupation and income) and geography (size of the urban unit and region). RESULTS: The component conditions of multimorbidity varied with gender and age. Women experienced multimorbidity 23-31% more frequently and at a younger age (5-15 years earlier) than men. Multimorbidity increased with age while its associations with most health indicators weakened with it. Multimorbidity was strongly and independently associated with socioeconomic indicators, with a strong inverse dose-response relationship with education, but less consistently with geographic factors. CONCLUSIONS: Multimorbidity has diverse and variable components and impacts across gender and age. It is strongly associated with socioeconomic factors, notably educational level, for which causality appears likely. Consideration of this diversity and variability, its common occurrence in dyads and triads, and its impact on health outcomes according to age and gender may contribute to efficient surveillance and support the identification of prevention strategies targeting middle-aged men and women.