Objectives: The aim of this study was to explore factors which mediated or moderated the effect of the Time2bHealthy online program for parents of preschool-aged children on body mass index (BMI) change. Study design: Mediation and moderation analyses of data from a two-arm parallel randomised controlled trial. Methods: Randomisation was conducted after baseline measures. The intervention group received an 11-week online program, and the comparison group received emailed links to information from an evidence-based parenting website. Data on the primary outcome (child BMI), potential mediators (energy intake, fruit and vegetable intake, discretionary food intake, physical activity, screen-time, sleep, child feeding, parent self-efficacy or parent role-modelling) and potential moderators (child age, parent age, parent income, parent education or parent living situation) were collected at baseline, 3 months and 6 months. PROCESS macro for SPSS was used to analyse possible mediators and moderators on BMI outcomes. Results: Despite significant food-related outcomes in the main analysis of this trial, no significant mediating or moderating effects were found for any hypothesised mediators or moderators. Conclusions: This study's null results could be explained by the high proportion of children in the healthy weight range, the study period not being long enough to detect change, the multicomponent nature of the intervention or the relatively small number of outcomes measured. Future childhood obesity interventions should continue to explore the effects of mediators and moderators on BMI and consider collecting data on a wide range of mediating and moderating factors to allow for comparison between studies to develop a better understanding of the factors contributing to successful interventions.