Sustainable food system transitions will likely require targeted dietary strategies, yet most models typically estimate environmental footprints at the aggregate level and overlook variation between individuals and contexts. Here we develop a globally harmonized model integrating individual food intake and supply chain data to estimate diet-related climate, water and land impacts, and dietary quality using the Alternative Healthy Eating Index (AHEI), across 72 socio-demographic subgroups in 165 countries. We find large disparities across groups. Highly educated urban men aged 15–34 years have the greatest emissions (median, 5.66 kgCO2e d−1), with 62% of their diets plant sourced. Low education, rural women aged 35–54 years have the lowest (2.85 kgCO2e d−1), with plant-sourced foods comprising 70% of their diets. Healthier diets are consistently associated with lower environmental impacts, with each 5% increase in AHEI associated with a 7.7% reduction in greenhouse gas emissions, a 4.4% reduction in water footprint and a 7.2% reduction in land footprint. Countries cluster into distinct patterns of diet quality and sustainability, indicating that targeted rather than uniform strategies are needed to accelerate sustainable dietary transitions.