<p>Young people transitioning from residential out-of-home care experience inequities across key life domains. To address this, transition planning is implemented for young people aged 15–17&#xa0;years. This evidence-informed practice aims to prepare them for adulthood by addressing their social, material, and developmental needs. However, the intended health and social outcomes are not consistently realised. Examining indicators of implementation fidelity may help explain these gaps, as lower fidelity is associated with poorer outcomes. Therefore, this study examined two indicators of fidelity: (1) adherence to key components of transition planning; (2) the quality of transition plans. A multi-source design drew on administrative data (<i>n</i> = 77), a staff adherence survey (<i>n</i> = 102), and transition plan documents (<i>n</i> = 95). Findings revealed notable gaps between intended and actual practice. Across the 18 transition planning components, mean adherence was 71.9% (<i>SD</i> = 11.85), indicating that while most components were delivered, implementation was inconsistent. Administrative data reflected similar gaps, with only 45.5% of young people having a transition plan in progress, and the quality of completed plans was modest (median = 58.3%). Significant differences in adherence and quality were observed across several demographic and contextual factors, indicating that implementation is shaped by the characteristics young people and staff and broader service system contexts. The study highlights the need for organisational and system supports that strengthen capability and promote greater consistency in practice. Further research should examine additional implementation outcomes and identify multilevel enablers and barriers to implementing transition planning in residential out-of-home care.</p>