Increasing concerns among mental health care professionals have focused on the impact of young people's use of digital technology and social media on their mental well-being. It has been recommended that the use of digital technology and social media be routinely explored during mental health clinical consultations with young people. Whether these conversations occur and how they are experienced by both clinicians and young people are currently unknown. This study aimed to explore mental health practitioners' and young people's experiences of talking about young people's web-based activities related to their mental health during clinical consultations. Web-based activities include use of social media, websites, and messaging. Our aim was to identify barriers to effective communication and examples of good practice. In particular, we wanted to obtain the views of young people, who are underrepresented in studies, on their social media and digital technology use related to mental health. A qualitative study was conducted using focus groups (11 participants across 3 groups) with young people aged 16 to 24 years and interviews (n=8) and focus groups (7 participants across 2 groups) with mental health practitioners in the United Kingdom. Young people had experience of mental health problems and support provided by statutory mental health services or third-sector organizations. Practitioners worked in children and young people's mental health services, statutory services, or third-sector organizations such as a university counseling service. Thematic analysis was used to analyze the data. Practitioners and young people agreed that talking about young people's web-based activities and their impact on mental health is important. Mental health practitioners varied in their confidence in doing this and were keen to have more guidance. Young people said that practitioners seldom asked about their web-based activities, but when asked, they often felt judged or misunderstood. This stopped them from disclosing difficult web-based experiences and precluded useful conversations about web-based safety and how to access appropriate web-based support. Young people supported the idea of guidance or training for practitioners and were enthusiastic about sharing their experiences and being involved in the training or guidance provided to practitioners. Practitioners would benefit from structured guidance and professional development to enable them to support young people in feeling more willing to disclose and talk about their web-based experiences and their impact on their mental health. This is reflected in practitioners' desire for guidance to improve their confidence and skills to safely support young people in navigating the challenges of the web-based world. Young people want to feel comfortable discussing their web-based activities during their consultations with mental health practitioners, both in tackling the challenges and using the opportunity to discuss their experiences, gain support, and develop coping strategies related to web-based safety.