Strengths-based working (SBW) is being adopted across multiple sectors, but the evidence base for its effectiveness is limited. In this assessment of a social outcomes contract, authors evaluated the impact of SBW on the delivery of homelessness-related outcomes for a high-risk client group. The novel impact evaluation design first identified characteristics in frontline staff associated with SBW and then decoded conjectural causal links between these characteristics and their clients’ social outcomes using qualitative comparative analysis. Data were collected from 65 frontline workers, measuring reflective practice, self-efficacy, and innovativeness. Authors found that clients of staff who most strongly exhibited characteristics associated with SBW achieved better long-term outcomes. This article makes four important contributions to our understanding of SBW. First, authors show how it is possible to undertake rigorous impact evaluation of SBW in a complex setting and identify moderately complex causal relations with limited sample size. Second, authors propose and test the presence of key characteristics associated with SBW and, in doing so, contribute to the theoretical understanding of SBW. Third, authors present new evidence on the effectiveness of SBW. Finally, the article demonstrates how social outcomes contracts can provide a framework for testing innovative social interventions.