Background: To offer optimal care, the mental health system needs new routes for collaboration, involving both interprofessional and interorganizational aspects. The transition from intramural to extramural mental health care has given rise to new dynamics between public and mental health care, introducing a challenge for interprofessional and interorganizational collaboration. This study aims to determine values and expectations of collaboration and to understand how collaboration in mental health care organizations takes shape in daily practice. Methods: We conducted a qualitative study using semi-structured interviews and a focus group, in the setting of the Program for Mentally Vulnerable Persons (PMV). Data were analysed following thematic analysis. Results: We found three aspect that were considered important in collaboration: commonality, relationships, and psychological ownership. However, our findings indicate a discrepancy between what is considered essential in collaboration and how this materializes in day-to-day practice: collaboration appears to be less manageable than anticipated by interviewees. Our data suggest psychological ownership should be added as value to the interorganizational collaboration theory. Conclusion: Our study offers a new definition of collaboration and adding “psychological ownership” to the existing literature on collaboration theory. Furthermore, we gained insight into how collaboration between different organizations works in practice. Our research points to a discrepancy between what all the partners find important in collaboration, and what they actually do in practice. Finally, we expressed ways to improve the collaboration, such as choosing between a chain or a network approach and acting on it and re-highlighting the goal of the Program Mentally Vulnerable persons.