Neighborhood parks are important venues for the urban population to do moderate-to-vigorous physical activity in leisure time. Parks can be particularly important for low-income neighborhoods, whose residents suffer from high rates of chronic diseases and may have less access to fee-based fitness exercise facilities. This study assessed the contribution of parks to local populations’ physical activity in 48 high-poverty neighborhoods in the city of Los Angeles, using systematic observation of park use and surveys of park users and residents conducted between 2013 and 2015. We found that parks accounted for approximately 2.1% (between-park SD = 1.4%) of moderate physical activity time and 3.1% (between-park SD = 2.1%) of vigorous physical activity time of the local population, both of which were notably lower than the city-level average previously reported. Parks’ contribution to physical activity was positively associated with park size (β = 0.13, p < 0.0001) and negatively associated with poverty (β = − 0.10, p < 0.0001) and local population density (β = − 0.25, p = 0.005). Parks in high-poverty neighborhoods in Los Angeles are underutilized, and more efforts are needed to fully realize their potential for population health.