The study aimed to test the efficacy of the core elements of the Health Action Process Approach (HAPA) in an intervention among parents to promote regular supervised toothbrushing of preschool-aged children. The pre-registered study (https://osf.io/fyzh3/) tested the effects of an intervention employing information provision, behavioural instruction, implementation intention and mental imagery techniques, adopting a randomised controlled design in a sample of Australian parents of preschoolers (N = 254). The intervention used an additive design with four conditions—education, self-efficacy, planning and action control—progressively layered to show the cumulative impact of incorporating self-efficacy, planning and action control strategies with a foundational education component. The intervention was delivered online, and participants completed self-report measures of parental supervised toothbrushing and HAPA-based social cognition constructs pre-intervention and 4 weeks post-intervention. Although no significant intervention effects on behaviour were observed, mixed-model analyses of variance (ANOVAs) revealed an increase in intention and task self-efficacy within the action control condition and an increase in action planning in both the action control and planning conditions from pre-intervention to follow-up. Despite no anticipated changes in behaviour, these findings endorse the use of theory- and evidence-based behaviour change strategies to inspire change in HAPA-based determinants of parental supervised toothbrushing: intention, action planning and task self-efficacy.