In Australia and internationally, co-responder models are becoming an increasingly common intervention to respond to police callouts where there is an assumed mental health-related component or crisis. This type of model involves a collaborative approach where trained police officers team with mental health clinicians to provide specialized responses in order to improve outcomes for persons with mental illness. However, there is limited understanding as to major elements required for implementation of the model. This study aims to identify the essential elements and challenges in implementation of the Mental Health Co-Responder Project in Cairns, Australia, where the team consists of a mental health nurse with demonstrated competencies in crisis intervention and a specially trained police officer. In 2016, 39 participants completed semi-structured interviews regarding knowledge and experience of the Cairns co-responder model. The participants represented first responders and community-based service providers who work with and support persons living with mental illness. Using a thematic analysis approach, key elements identified as essential to successful project implementation were as follows: co-responder team characteristics, senior and project executive level support, collaborative project governance, and co-location of the team within a mental health setting. The main perceived challenges to project implementation included the following: initial concerns regarding client confidentiality, lack of an evaluation plan, and adequate project resourcing. Governance through a vigorous joint agency operation committee and adequate resourcing is imperative to the sustainability of this model.