Empirical urban design research emphasizes the support in vitality of public space use. We examine the extent to which a public space intervention promoted liveliness and three key behaviors that enhance well-being ("connect," "be active," and "take notice"). The exploratory study combined directly observed behaviors with self-reported, before and after community-led physical improvements to a public space in central Manchester (the United Kingdom). Observation data (n = 22,956) and surveys (subsample = 212) were collected over two 3-week periods. The intervention brought significant and substantial increases in liveliness of the space and well-being activities. None of these activities showed increases in a control space during the same periods. The findings demonstrate the feasibility of the research methods, and the impact of improved quality of outdoor neighborhood space on liveliness and well-being activities. The local community also played a key role in conceiving of and delivering an effective and affordable intervention. The findings have implications for researchers, policy makers, and communities alike.