One core component of the Paediatric Autism Communication Therapy–Generalised involves supporting parents to change their interaction and communication style with their child. This behaviour change has been found to affect child outcomes. Implementation science methodologies offer a range of opportunities to investigate how interventions are delivered in practice; however, few autism intervention studies have used such designs to explore the behaviour change of parents. We interviewed 27 parents and explored their use of intervention strategies after the Paediatric Autism Communication Therapy–Generalised trial. We employed the Theoretical Domains Framework and Consolidated Framework for Implementation Research to systematically explore a range of barriers and facilitators experienced by the parents associated with parents’ characteristics, their context and features of the intervention. Our analysis revealed barriers and facilitators across three themes: Motivating Factors, which was further subdivided into Compatibility and Buy-In and Alignment of Goals and Outcomes; Opportunity and Support; Parent Characteristics. Almost all parents reported continued use of the Paediatric Autism Communication Therapy–Generalised strategies. Facilitators (e.g. parental confidence in using the strategies) and barriers (e.g. child’s behaviour) were identified. Consideration of these factors can inform identification of implementation strategies to test in future studies of Paediatric Autism Communication Therapy–Generalised and other parent-mediated early autism interventions. Lay abstract Many early autism interventions teach parents therapeutic strategies to help them adjust their communication style with their children. Research has shown that this behaviour change in parents leads to improvements in child communication. It is, therefore, important to learn what factors support or hinder parents in their use of therapeutic strategies learned in such interventions. This study set out to interview parents who had participated in a research trial of the Paediatric Autism Communication Therapy–Generalised intervention. We interviewed 27 caregivers and explored their use of the strategies up to 2 years after the end of the research trial. Qualitative frameworks were used to inform interview questions and data analysis. These frameworks focused on a range of contextual factors, including parents’ characteristics, their context and features of the intervention. Parents reported barriers and facilitators to using Paediatric Autism Communication Therapy–Generalised strategies across three themes: Motivating Factors; Opportunity and Support; Parent Characteristics. One of these themes, Motivating Factors, was further divided into the subthemes Compatibility and Buy-In and Alignment of Goals and Outcomes. Almost all parents reported continued use of the Paediatric Autism Communication Therapy–Generalised strategies. Facilitators included parental confidence in using the strategies and barriers included child’s behaviour. Consideration of these factors can inform ways to better support parents in future autism interventions.