Age-related changes can affect mental health, but aging-focused mental health research is limited. The objective was to identify the top 10 unanswered research questions on aging and mental health according to what matters most to aging Canadians. A steering group of experts-by-experience (e.g., older adults, caregivers, health and social care providers) guided three phases of a modified James Lind Alliance priority-setting partnership: (1) a broad national survey (n = 305) and a rapid literature scan; (2) a follow-up national survey (n = 703); and (3) four online workshops (n = 52) with a nominal group technique. Forty-two unique questions on aging and mental health resulted, of which 18 were determined to be answered by existing evidence. Of the 25 partially and unanswered questions, 10 were ranked as top priority. Findings can be used to prioritize future research, knowledge mobilization, and funding decisions, and to promote and support collaboration between longstanding siloed research and care fields.