There is emerging evidence on the value of interprofessional education (IPE) for health professional students; however, there is less focus on assessing students on IPE experiences in clinical placement contexts. Researchers have focused on student attitudes to, or readiness for, IPE. Multiple other elements need to be assessed, including formative student assessment, profession-specific competency standards, and students’ interprofessional (IP) capabilities. Effective and authentic IP-assessment models are needed that can both demonstrate student learning, and facilitate successful sharing of assessment amongst professionals. Supervisor’s perspectives on successful sharing of assessment amongst professions have not been addressed to date in the literature. This article describes one approach for IP-Assessment, using a complex theory framework and based on an iterative cycle of developing, implementing, and evaluating a student assessment tool within an international IP placement, which could be utilized in a range of settings. A qualitative research approach explores the strengths, challenges, and outcomes of IP-Assessment from the perspective of the supervisors. Three overarching themes were identified: acknowledging the student journey, interprofessional authenticity, and collective and collaborative learning. The process of developing and implementing IP-Assessment enabled the articulation of placement and assessment priorities and provided learning opportunities for both supervisors and students.