This paper shows how system-level data can generate useful insights into the profile of first-year graduates who are teaching out-of-field (OOF). Understanding in-school demand and impacts on first-year graduates teaching OOF is important, especially when the first years of teaching are complex, busy, and involve a steep learning curve. A mixed methods sequential explanatory design was used to generate qualitative and quantitative data collected from teacher and principal surveys. This study establishes the prevalence of OOF teaching amongst first-year graduates in Western Australian public schools and develops a profile of their OOF experiences and support needs based on their perceptions and their principals' perceptions. This study details the supports that will make a difference to teachers' capability, in particular a reduction in load where the OOF teaching workload is high, and access to formal subject-specific mentoring, both noted in research as important support mechanisms. This paper provokes new research questions, highlights areas for further research, and establishes the need for more large-scale datasets and longitudinal studies to examine change over time and the enduring effects of OOF teaching, both in relation to transitions into teaching, as teachers learn on-the-job, and when teachers undertake formal professional development programmes or courses.