Aim <p>Young adulthood is a formative life stage during which modifiable behaviours, including diet and physical activity (PA), can have lasting impacts on brain health. However, this age group remains understudied in dementia research. This study aimed to explore how younger adults track their diet and PA, and evaluated attitudes, barriers, and enablers to different assessment tools.</p> Subject and methods <p>An online questionnaire assessed diet and PA tracking behaviours, attitudes, and barriers/enablers in younger adults (18–39&#xa0;years) across the UK and US. Responses were compared between countries, ages, sexes, and ethnicities.</p> Results <p>One thousand and six younger adults (UK <i>n</i> = 500, US <i>n</i> = 506) participated, with 90.3% reporting they would be likely/very likely to participate in a study exploring lifestyle and brain health. Remote technology-based data collection methods, particularly apps and smartwatches, were widely acceptable. Most participants were willing to provide annual dietary and activity data. Key diet-tracking barriers included estimating portion sizes and tracking outside the home. Key PA-tracking barriers included day-to-day variability and forgetting to record activity. Enablers included receiving incentives and using passive tracking methods. Participants from the US, of a minority ethnic group, or aged 18–29&#xa0;years reported greater barriers to tracking.</p> Conclusions <p>Younger adults are interested in participating in brain health research and find technology-based diet and PA tracking acceptable in this context. Addressing identified barriers will be key to building a diverse, scalable cohort. Pilot testing is now needed to optimise feasibility and engagement. These findings will inform the design of a future brain health-focussed cohort study.</p>