Background: People with intellectual disability experience high rates of mental ill health but multiple barriers to access to quality mental health care. One significant barrier to access is a generalist mental health workforce that lacks capacity, and consensus on what constitutes core workforce competencies in this area. As such, the first step in developing a comprehensive strategy that addresses these barriers is to define the core mental health workforce attributes. Methods: Thirty-six intellectual disability mental health experts from across Australia participated in a modified online Delphi which aimed to reach consensus on the core attributes required of the generalist mental health workforce. At the end of each Delphi round, descriptive analyses and thematic analyses were completed. Results: Consensus was reached among the participants for 102 attributes across 14 clinical domains. The results highlighted specific attributes in all areas of clinical practice, especially for communication (e.g., utilizes assistive communication technology), assessment (e.g., assess contributing factors to behaviours) and intervention (e.g., uses integrative interventions). Conclusion: A comprehensive strategy is required to facilitate the generalist mental health workforce to obtain these attributes.