The use of mixed forests and non-native tree species have the potential to mitigate climate change impacts and enhance biodiversity. However, little is known about how forest composition and environmental factors affect each step of natural regeneration in mixed forests, especially in mixtures with non-native trees. Here we investigated how forest composition affected European beech seed survival (through seed tracking), seed sprouting (via field germination experiments), and seedling survival (monthly seedling monitoring) in pure beech forests and in admixtures with Norway spruce and the introduced Douglas-fir in a mast and non-mast year of beech. We also assessed how biotic and abiotic factors (small mammal abundance, ungulate access, seed production, seed burial, canopy cover, distance to nearest adult tree, seedling aggregation, understory density, browsing damage, and soil properties) affected these regeneration dynamics. Seed survival was negatively affected by the presence of conifers and mouse abundance. Seed germination was influenced by whether seeds were buried or not. Seedling survival increased in Douglas-fir admixtures, and in forests with higher soil quality. Browsing damage and ungulate access diminished seedling survival. Seed production had the greatest influence on absolute number of seedlings. Forest composition and environmental factors had distinct impacts on regeneration of beech depending on its ontogenetic stage. Here, we provide evidence supporting the notion that Douglas-fir is not impairing the regeneration of native trees in mixed forests. In fact, mixtures with Douglas-fir benefited the survival of beech seedlings, likely due to better soil properties and less damage from herbivores on these stands.