Objective: The purpose of this longitudinal study was to examine the effects of state-level socioeconomic status (SES), the density of fast food restaurants and walking to work on body mass index (BMI) among US adults aged 50 years and older. The study sought further to account for the interaction effects of three different hierarchical levels of variables, including time-varying variables and time-invariant variables such as individual and state-level variables. Confounding related to environmental/neighbourhood effects was controlled for through sample selection. Methods: Data were drawn from the 2000-2010 Health and Retirement Study (HRS) and the 2010 US Census. HRS survey participants aged 50 years and older in 2000, residing in the same census tract during 2000-2010 (N = 6,156) comprised the study sample. A three-level growth model was fitted to BMI trajectories. Results: BMI significantly increased over time showing both a linear and quadratic decelerating trajectory (p < 0.001) where BMI grew faster among sampled adults in their early 50s. Women living in the US states with a lower state-level SES had higher BMIs, while men with higher state-level SES had a higher BMI (b = -0.242, p = 0.013). In states with a higher proportion of people who walked to work, Hispanics had higher BMIs at baseline and a lower BMI growth rate, compared with non-Hispanic whites (b = -0.033, p < 0.001). Conclusions: Obesity interventions specific to adults aged 50 years and older require greater emphasis. Potential health inequalities regarding weight gain can be ameliorated through tailoring interventions based on sex, environmental and state-level SES influences.