Background: Young adults (18–25) face significant risk for weight gain and transitioning to a higher body mass index category when compared to other adult groups. As active internet users, they encounter food-related content across digital platforms, yet little is known about their behavioural and cognitive responses to this compound exposure. Methods: This pre-registered mixed-methods systematic review features primary studies about participants aged 18 to 25 exposed to digital food communication and assessed for behavioural or cognitive responses towards food and nutrition. We evaluate consumption and food purchase as behavioural responses; intentions to consume and/or purchase, and attitudes towards food and nutrition as cognitive responses. We searched PubMed, PsycINFO, Global Index Medicus, and Business Source Ultimate for studies published from database inception to August 1, 2024. Quality appraisals were conducted using a modified version of the Newcastle-Ottawa Quality Assessment Form, the ROB1 Tool for randomised trials, the JBI Quasi-Experiment Checklist for quasi-experiments and the Critical Appraisal Skills Programme (CASP) checklist for qualitative studies. We used a three-pronged method for analysis. Meta-analyses combined findings from randomised trials for behavioural and cognitive responses, while observational studies were summarised narratively. The thematic synthesis approach informed our qualitative synthesis of young adults’ narratives of their responses after exposure to digital food communication. Finally, a cross-study matrix synthesised analytical qualitative themes and quantitative findings. Results: Of the 6132 studies identified, 45 are included in the systematic review, representing 8,914 young adults in 16 countries. Meta-analyses on behavioural and cognitive responses demonstrate statistical significance with effect sizes of 0.34 (95% CI: 0.18-0.50) and 0.19 (95% CI: 0.10-0.28), respectively. Observational studies confirmed the findings of the meta-analyses. Difficulty deciphering what represents good nutrition, critical distinctions when engaging with content viewed as helpful versus misleading and balancing intentions versus actual behaviours were barriers to the effectiveness of digital food communication. Using a cross-study synthesis matrix, we developed ten recommendations to improve digital dietary interventions and their assessed implementation by experimental studies in the review. Conclusions: Our results illustrate the need to approach digital food communication as a digital determinant of dietary health for young adults, shaping behaviours and cognition.