Understanding the factors that facilitate trust and collaboration in Mobility as a Service (MaaS) ecosystems is crucial to the effective delivery of a MaaS scheme given the involvement of both public and private stakeholders. This paper presents the findings of a study investigating the factors that build trust and collaboration among the stakeholders in a MaaS ecosystem and provides insights into the role of regulatory authorities in enabling MaaS developments. The research approach employed the Constructivist Grounded Theory Method (CGTM) using the distinct cases of Whim in Helsinki, Finland and The Sydney MaaS trial in Sydney, Australia. Semi-structured in-depth interviews were conducted with the representatives of key stakeholder organizations in the two MaaS ecosystems. A conceptual framework of trust and collaboration was developed for each case study to illustrate the process of forming and consolidating trust and collaboration in each MaaS ecosystem. The findings suggest that trust acts as the antecedent and catalyst for building collaborative space within a MaaS ecosystem. Possessing a good understanding of partners’ expectations is a prerequisite for collaboration given the inherent conflicts between interests and goals of the public and private sector. The capability of the partners, prior relationships and open negotiations emerged as the key constructs which form trust in a MaaS ecosystem. It was evident that appropriate policies and legislation developed with the consultation of stakeholders are critical in the development of trust and collaboration in the MaaS ecosystem.