Background: Parent engagement in child-focused interventions is increasingly recognised as an important aspect of effective intervention delivery. While several fields have an emerging literature around parent engagement, no reviews currently exist which combine findings across allied health literatures. Objective: This review aimed to explore factors relevant to understanding parent engagement in child-focused interventions, as described in qualitative literature across allied health disciplines, toward informing the clinical practice of helping professionals in effectively engaging parents. Methods: A systematised qualitative literature review was carried out, with a comprehensive search of five online databases (CINAHL, Embase, MEDLINE, PsycINFO, Scopus) for allied health literature (specifically: behaviour analysis, occupational therapy, psychology, and speech-language therapy) using parent engagement key words. Reference searching and citation tracking steps supported the search. Thematic synthesis was used as the overarching framework and analysis approach. Results: 8824 unique studies were generated in the search. Of the 71 studies which met inclusion criteria, 38 reported qualitative findings and were included in the analysis. Five themes were identified including: societal context, interpersonal context, clinician features, family features, and relationship as engagement. Conclusions: Findings support conceptual explanations of parent engagement as a complex and dynamic process, emphasising the joint contributions of parents and clinicians in developing therapeutic relationships which promote engagement. Across allied health research there are consistency of understandings around parent engagement, supporting the conclusion that clinicians can look to literatures from various helping fields to inform clinical practice around engaging parents in interventions.