The Delphi method has been previously used to define and reach consensus on core workforce attributes. However, limitations of the traditional Delphi method have been identified. A modified online Delphi method, which aims to address these limitations, was used as one part of the intellectual disability mental health core competencies project. The authors assess the utility of a modified online Delphi method to define and reach consensus on the core intellectual disability mental health workforce attributes and the lessons learned from using this approach. Thirty‐six intellectual disability mental health experts from across Australia participated in a modified online Delphi. At the end of each Delphi round the research team completed descriptive analyses for the quantitative data, and content and thematic analyses for the qualitative data. The research team subsequently critiqued the method in terms of its utility in defining workforce attributes. Authors identified several lessons from using this method to define core workforce attributes. In particular, it has highlighted the potential benefit of initiating the Delphi process with a developed framework that is familiar to the workforce; the benefit of using quantitative and qualitative methods; and the need to reconsider how to define participant consensus. This study confirmed the utility of a modified online Delphi method to identify and describe core workforce attributes, provided insights into the benefits and challenges of this method, and identified strategies for overcoming some of the commonly documented limitations of the Delphi method. However, it has also highlighted some additional challenges of the Delphi method that researchers have not commonly described within the published literature. The lessons provide useful insights for others planning to use the Delphi method.