Context. Many challenges to implementing the collaborative care model are related to adopting the care manager's role. Little is known about the determinants of practice related to current nursing and collaborative care practices that might be targeted by implementation strategies to improve the adoption of care manager activities when implementing the collaborative care model for people with common mental disorders (CMDs) and physical conditions. Objective. Evaluate the main determinants of practice influencing the adoption of care manager activities by primary care nurses for people with CMDs and physical conditions. Design. A qualitative multiple case study using an integrated knowledge translation approach. Setting. Three family medicine groups (FMG) in Quebec, Canada. A total of 33 participants were recruited according to a purposeful sampling: eight patients, 13 primary care nurses, four general practitioners, three social workers, two nurse practitioners, two psychologists, and one pharmacist. Data collection. Interviews (n=32), observations of a nurse-patient encounter (n=7), internal documents, and summary of meetings with key stakeholders (n=8). Analysis. An intra-case analysis was first conducted, followed by an inter-case analysis. An assessment of the gap between the care manager activities and primary care nurse’s activities was realized using an analysis table. Results. Determinants are classified into four primary levels: mental health care system, FMG characteristics, collaboration among team members, and nurse characteristics. Emerging determinants were related to (1) access to external mental health resources, (2) clarification of the local CMDs care trajectories, (3) compatibility of the coordination of nursing work with the role of the care manager, (4) availability of mental health resources within the primary care clinic, (5) competencies in care management of people with CMDs and competency-building, (6) responsibilities sharing between the general practitioner and the primary care nurse, and (7) common understanding of the patient treatment plan. Conclusions. Identifying determinants of practice before implementing the collaborative care model and the care manager's role appears necessary to select implementation strategies tailored to current practices. Further researches are needed to evaluate performance indicators related to determinants of collaborative care and nursing practices, mainly organizational related determinants.