An Intensive Case Management (ICM) intervention has been developed in Lausanne, Switzerland. It aims to promote access to care for people with severe mental disorders who have difficulties to engage with mental health services because of the severity of their disorders and/or their marginality. ICM embrace components of assertive community treatment and critical time intervention. It is time limited and focused on critical phases of recovery. The goal of this study was to examine the heterogeneity of service use patterns of people who required ICM interventions and identify differences in patterns of duration and timing of the intervention. Records of 471 patients from the Department of Psychiatry of Lausanne University Hospital for whom the ICM team intervention was requested were analysed over a 6 year period with discrete sequential-state analysis. Trajectories could be split between six meaningful clusters including service light use and critical time intervention (58.0%), transition to long-term regular ambulatory-care (11.3%), partial transition to ambulatory care (14.4%), alternative to hospitalization (10.4%), continued ICM (4.9%) and long hospital stays (1.1%). Diagnoses of substance abuse were overrepresented among heavy users and diagnoses of schizophrenia were the most frequent diagnostic overall. Profiles of service use for ICM patients were very diverse. Long term interventions were frequently not necessary. A time-limited intervention was likely sufficient to stabilize the situation and/or engage the patient in care. A small number of situations required a sustained and long-term investment and did not always allowed for a reduction in the need for hospitalization. A general reflection on alternatives to hospitalization must be pursued, in particular for these patients.