Mobile applications (apps) have the potential to improve mental health services. However, there is limited evidence of efficacy or responsiveness to user needs for existing apps. A lack of design methods has contributed to this issue. Developers view mental health apps as stand-alone products and dismiss the complex context of use. Participatory design, particularly an informed participation approach, has potential to improve the design of mental health apps. In this study, we worked with young mobile users and mental health practitioners to examine the informed participation approach for designing apps. Using auto-ethnography and a set of design workshops, the project focused on eliciting design requirements as a factor for successful implementation. We compared resultant ideas and designs with existing apps. Many user requirements revealed were absent in existing apps, suggesting potential advantages to informed participation. The observation of the process, however, showed challenges in engagement that need to be overcome.