Objectives: To evaluate the effects of digital health intervention (DHI) or/and personalized healthy breakfast guidance (PHBG) on the breakfast behavior and body composition of young adults in Tianjin, and to explore the underlying behavioral mechanisms using the Health Action Process Approach (HAPA) framework. Methods: In this single-blind, stratified RCT, 160 participants (n = 40/group) were randomly assigned to a control group, DHI group, PHBG group, or DHI + PHBG group. Breakfast behavior (primary outcome), HAPA constructs, and body composition were assessed at baseline and after 1 month. Group differences were analyzed using the Kruskal–Wallis test, chi-square test, and linear mixed-effects models. Mediation analysis assessed indirect effects via HAPA variables. Results: After a 1-month intervention, adherence to healthy breakfast guidelines was highest in the DHI + PHBG group (80%), followed by the PHBG (72.5%) and DHI (50%) groups, compared to 7.5% in the control group (χ² = 51.127, p < 0.001, DHI + PHBG group > DHI group: χ² = 7.912, p < 0.05). All interventions advanced participants along HAPA stages (H = 34.678, p < 0.001) and improved self-efficacy and planning. PHBG and DHI + PHBG further enhanced outcome expectations, intention, and, for the DHI + PHBG group, self-monitoring. Self-efficacy mediated 17.636% of the PHBG effect and 13.305% of the DHI + PHBG effect, and self-monitoring mediated 7.401% of the DHI + PHBG effect. Waist-to-hip ratios decreased modestly in all intervention groups (β = −0.015 to −0.013, p < 0.05), but no significant changes were observed in other body composition indices. Conclusions: DHI, PHBG, and especially their combination, improved breakfast habits in young adults, with self-efficacy as a key mediator. However, the effects of these interventions on body composition were limited due to the short duration of the trial.