Studies of the built environment and physical activity (PA) have primarily been cross-sectional. Evidence on the causal impacts of transportation improvements on PA and sedentary behavior (SB) is lacking. This study assessed the effect of retrofitting an urban greenway on PA and SB in Vancouver, Canada. A sample of 524 participants (median age of 44; 57% female) were divided into experimental and control groups, and the effect of exposure to the greenway was examined by using different distance thresholds. Self-report measures of moderate-to-vigorous PA (MVPA) and SB were collected using the International Physical Activity Questionnaire (IPAQ-SF) before (baseline; 2012–2013) and after (follow-up; 2014–2015) construction of the Comox-Helmcken Greenway in 2013. Mixed-effects models estimated the impacts of greenway on MVPA and SB. For participants living near the greenway (≤300 m), the odds of achieving an average of 20 min of daily MVPA doubled (OR = 2.00; 95% CI = 1.00, 3.98) after the greenway's opening. The odds of being sedentary for >9 h declined by 54% (OR = 0.46; 95% CI = 0.25, 0.85) after opening. PA benefits from the greenway declined with distance from 100 m to 500 m. Reduction in SB was lowest at 100 m and greatest at 300 m. Retrofitting an urban residential neighborhood through greenway interventions can be successful in promoting physical activity while reducing sedentary behavior. Recommendations for future longitudinal research include the use of objective PA measures, studying different neighborhood contexts, collecting more representative samples, and minimizing attrition.