The circular economy in the construction sector aims to lessen the environmental impact of construction projects by incorporating the end-of-life phase into the design and the conscious use of reusable resources. To achieve a circular economy in the built environment, it has been evidenced in the literature and practice that governance mechanisms in inter-organizational collaboration are required between the multiple parties involved to quickly rethink the purpose of the project, the desired outcome, and the aligned efforts towards the solutions of circular challenges. This research gains insight into the governance mechanisms that have proven to be desirable or beneficial in the collaboration between public clients and main contractors during the front-end phase in building projects with circular values. It is accomplished through two stages: (1) a comprehensive literature review was conducted where 25 articles were selected based on keywords related to inter-organizational collaboration. It brought the most representative knowledge of the existing forms and types of collaboration on construction projects, as well as an existing framework created to analyze the approaches for coordination, adaptation, and safeguarding of exchanges between the focal parties of inter-organizational projects. The literature included a second search of current circular practices in construction projects; (2) a multiple-case study strategy was conducted to explore how the inter-organizational collaboration between public clients and contractors was governed toward the delivery of circular building projects. Data was collected from publicly available documents and semi-structured interviews on the three cases that shared the most similar approach based on applied circular practices. This study's result contributes to the current literature and practice by asserting that governance mechanisms in inter-organizational collaboration between public clients and main contractors facilitate the adoption and implementation of circular practices in building projects. The results expressed that relational and contractual governance mechanisms complement one another in goal-setting, capability building, roles and decision-making, rewarding, coordination, and monitoring. Various governance mechanisms within each of these dimensions can be employed to pursue circular practices under specific contexts, thereby supporting the transition to a circular economy in building projects.