Background: The prevalence of child and adolescent mental health issues are rising faster than the number of services available, leading to a shortfall. Mental health chatbots are a highly scalable way to address this gap. Manage Your Life Online (‘MYLO’) is an artificially intelligent chatbot that emulates Method of Levels Therapy (MOL). MOL is a style of therapy that uses curious questioning to support sustained awareness and exploration of current problems. Objective: To assess the feasibility and acceptability of a co-designed interface for MYLO in young people aged 16-24 with mental health problems. Methods: An iterative co-design phase occurred over 4 months, in which feedback was elicited from a group of young people (N = 7) with lived experience of mental health issues. This resulted in the development of a progressive web application version of MYLO that could be used on mobile phones. We then conducted a case-series to assess feasibility and acceptability in 13 young people over 2 weeks. During this time the participants tested MYLO and completed surveys including clinical outcomes and acceptability measures. We then conducted focus groups and interviews, and used thematic analysis, to obtain feedback on MYLO and identify recommendations for further improvements.Results: Most participants were positive about their experience of using MYLO and would recommend MYLO to others. Participants enjoyed the simplicity of the interface, found it easy to use, and rated it as acceptable on a system usability scale. Inspection of the usage data found evidence that MYLO can learn and adapt its questioning in response to user inputs. We found a large effect size for the decrease in participants’ problem-related distress and a medium effect size in the increase in their self-reported tendency to resolve goal conflicts (the proposed mechanism of change) over the testing phase. Some patients, also, experienced reliable change in their clinical outcome measures over the two-weeks.Conclusions: We have established the feasibility and acceptability of MYLO. Initial outcomes suggest MYLO has the potential to support young people’s mental health and help them to resolve their own problems. We aim to establish whether the use of MYLO leads to meaningful reductions in participants’ symptoms of depression and anxiety, and whether these are maintained over time, by conducting a randomised controlled evaluation trial.