The design of integrated adolescent mental health care should address needs and preferences of patients and parents/guardians. We conducted interviews and focus groups with adolescents aged 13-17 years who received care at Kaiser Permanente Washington in 2020 and interviews with parents of such adolescents. We sought to (1) understand the challenges of primary care-based mental health and substance use screening and care for adolescents and (2) identify program design solutions. Sessions were audio-recorded, transcribed, and coded. Thematic analysis was applied to identify key challenges and design solutions. Emerging themes from interviews (n=41) and focus groups (n=10) were summarized in five overarching design principles: Engagement, Privacy, Communication, Choice, and Ease. Each design principle was expanded for operationalization within a new health system program. Health systems serving adolescents in primary care may consider application of these design principles to the development of mental health integration programs.