Marginalisation is a multilevel phenomenon in society depriving people from essential rights, resources, and opportunities. Street-outreach services in the Netherlands, like social street work (SSW), support these marginalised people in fostering their participation in society as an answer to their marginalised position in society. We followed 927 clients in SSW over an eight-month period. Clients filled in a questionnaire at three timepoints. We examined whether clients’ perceived belongingness, self-esteem, strengths, and informal support (outcome measures) were associated with the working relationship, over time. Results showed the establishment of a working relationship with clients at all three timepoints. An evolving working relationship was associated with an increase in clients’ perceived belongingness, self-esteem, strengths, and informal support over time. This study showed the ability of workers to establish a working relationship with clients in their living environment and underscored the necessity of establishing a working relationship in street-outreach services to foster clients’ participation in society. This study encourages policymakers to reflect on current street-outreach services, to deviate from demanding short-term and measurable results from professionals’ efforts, and to opt for a better fit between performance and financing conditions and daily practices of street-outreach services.