A neighborhood’s built environment can challenge the mobility of older mobility-challenged adults (due to knee osteoarthritis and falls), reducing their participation and quality of life. The Built Environment in Falls and arthrITis (BE-FIT) study aims to understand the neighborhood influence on the mobility, participation, and psychosocial health of older adults with knee osteoarthritis and/or falls. BE-FIT comprises four work packages (WPs). WP1 quantitatively explores relationships among environmental-, social-, and person-related factors and participation outcomes of its intended population. WP2 employs qualitative methods to comprehend the relationships among WP1’s variables. Via a combination of wearable sensor technology and qualitative geospatial methods, WP3 aims to characterize its population’s movement behavior, mobility, functional activity of daily living, and lived experiences of residing in a mature neighborhood. Finally, WP4 engages crucial stakeholders to co-develop evidence-based recommendations to inform public health, urban planning, and aging policies and implementation. BE-FIT could benefit societies with rising incidence of knee osteoarthritis and falls by improving neighborhoods and lives of older mobility-challenged residents.