Flexible and integrative treatment (FIT) services in Germany make it possible to shift mental health inpatient care to day- and outpatient care. This paper presents the results of the mixed methods, participatory process evaluation (= PE) of the PsychCare clinical trial, which compares the outcomes of nine FIT departments to those of eight standard mental health care services. This PE integrates diverse data using a program theory and data transformation approach. It shows various implementation types of FIT services and how their processes and structures are distinct from those of control conditions, also experienced by service users. It contributes to mixed methods research by showing that the PE methodological frame is a valuable tool to effectively involve service users and other stakeholders.