The study examined whether routine outcome monitoring may be a tool that can help improve floating support services aimed at preventing homelessness among vulnerable citizens. We evaluated the effect of routine outcome monitoring using the Partners for Change Outcome Management System (PCOMS) in a cluster randomized controlled trial. PCOMS monitors the progress of the well-being of the client and the working alliance between the therapist and client in order to customize the treatment to the individual client. In all, 530 clients were treated by 42 support workers in the treatment group and 490 clients were treated by 38 support workers in the control group. The outcome measures included clients’ well-being, health and social relations, whether clients benefitted from the service, the duration of the service and whether the clients were at risk of being evicted from their tenancy. We found no statistical difference in outcomes between the treatment group and the control group for any of the included outcomes. It was difficult for the support workers in the floating support service to use PCOMS in every meeting with their clients, and we did not find an effect of PCOMS for the clients in the floating support service.