Preventing and reducing youth violence remains a significant public health challenge for urban marginalized communities across the country. This project used a Communities That Care (CTC) approach to build a community-level prevention infrastructure across two Denver, Colorado communities experiencing high-burden, with a primary aim of reducing youth violence. To examine the impact of the CTC system on youth violence outcomes, we assessed within- and between-community effects using two quasi-experimental approaches—Interrupted Time Series (ITS) and Difference in Differences (DiD) analyses with synthetic controls. The primary violence outcome was official youth (aged 10–24) annual arrests for violent offenses, measured for five years prior to the intervention (2012–2016) and for five years after the start of the intervention (2017–2021). ITS results show that there was an intervention effect in Community B as demonstrated by lower arrest rates early in the intervention period, suggesting that the mobilization period was impacting the intervention community with respect to the arrest rates. There were no significant effects in Community A. The DiD analysis with a synthetic control confirmed that the arrest rates in Community B were significantly lower than in the synthetic control community during the intervention years. The CTC approach and the community-level prevention system implemented in intervention Community B was associated with a reduction in youth arrests for violent offenses during the intervention period; no intervention effects were detected in Community A. Additional research is needed to understand why the intervention was effective in Community B, but not Community A.