Poor diets are a leading cause of morbidity and mortality. Exposure to low-quality food environments, such as ‘food deserts’ with limited access to grocery stores and ‘food swamps’ saturated with fast food outlets (FFO), is hypothesized to negatively impact diet and related disease. However, research linking such exposure to diet and health outcomes has generated mixed findings and led to unsuccessful policy interventions. A major research limitation has been a predominant focus on static food environments around the home and workplace, and sparse availability of information on the dynamic food environments people are exposed to and the food outlets they visit as they move throughout the day. We leverage population-scale mobility data – anonymized GPS trajectories from people’s smartphones, collected from approximately 15 million U.S. adult residents – to examine peoples’ visits to food outlets and FFO in and beyond their home neighborhoods. First, we show that visits to FFO provide strong and significant indicators of FF intake and diet-related disease. Second, we develop a semi-causal framework using several natural experiments to evaluate how food choice is influenced by features of food environments people are exposed to in their daily routines vs. individual preference. We find that individuals in large urban areas habitually travel considerable distances to food outlets (median: ~8km), consume most fast food away from home during lunch on workdays, and experience greater dietary and health impact from FEs outside their home neighborhood. These findings reveal a critical research gap in understanding how at-risk populations interact with FEs beyond their home neighborhoods in the context of their lifestyle routines, and the causal effects of these FE exposures on food choices — insights pivotal to designing food environment interventions that meet people when and where they are needed. Through simulation, we explore example intervention strategies to illustrate the value of accounting for dynamic food environments. In ongoing work, we have partnered with the Los Angeles County Department of Regional Planning and Department of Public Health to use these insights to directly guide the design of implementable interventions to food environments that optimally increase equitable access to healthy foods.