Background: Young people who have a parent with a mental illness face elevated risks to their wellbeing. However, they may not have access to appropriate interventions. Online interventions may reach and meet the needs of this at-risk group, yet their preferences regarding the features of this medium are unknown. Objective: This study sought to determine the utility of an online intervention to meet the needs of young people who have a parent with a mental illness, and their perspectives regarding the types of features of such a website. Methods: A systematic, two-round Delphi study was employed to solicit the views of 282 young people aged 16 to 21 years (Round 1 n = 14, Round 2 n = 268), from urban and regional settings in Australia. 'Regional' was used to refer to non-urban participants in the study. After first ascertaining whether an online intervention was warranted, an extensive list of online intervention features was identified, including how the site might be facilitated, topics, duration and frequency, and the nature of professional contact. The extent to which young people agreed on the importance of these factors was assessed. Differences and similarities across gender and location were investigated. A mixed method analytic framework was employed using thematic analysis as well as two-way betweengroups analysis of covariance controlling for age (ANCOVA) and chi-square test of independence analysis. Results: Both rounds highlighted a strong preference for an online intervention. Consensus was reached for 1) a professionally monitored site; 2) young people and professionals having equal input into the weekly facilitated sessions (e.g., sharing lead role in discussions or deciding on relevant session content); 3) unlimited time access; 4) one-hour, open discussion, weekly sessions over 6 weeks; 5) preferred features for an online intervention; 6) psychoeducation about mental illness; and, 7) considerations for the management of safety violations. There were significant main effects of location type and several of the preferred features for an online intervention for young people who have a parent with a mental illness; however, effect sizes were small to moderate, limiting practical application. Conclusions: Young people aged 16 to 21 years indicated a need for a professionally monitored, psychoeducational, online intervention, with input from professional facilitators and other similar young people, in addition to recommendations to external resources. The findings can be used to inform the development of future online interventions for this highly vulnerable group.