In this article, we contribute to understanding of the mechanisms through which students' socio-economic family background can translate into academic performance by focusing on the concept of student engagement. Drawing on theoretical perspectives from disciplines across the social sciences, and a major nationally representative dataset from Australia, which links survey responses with administrative records on school performance, we conduct a series of multiple regression models to investigate the mediating role of student engagement on the relationship between students' socio-economic status (SES) and academic achievement. We find that, first, low-SES students show lower levels of engagement than other students, particularly in terms of behavioural and cognitive aspects; they also have lower achievement levels as measured by standardised test scores. We further find that lower engagement is associated with lower achievement levels, and that the effects of SES on achievement are partially mediated through student engagement. Although there are clearly other mechanisms in place that operate at the same time, it is important to focus on student engagement since it can be directly influenced by teachers and school leaders, as well as curriculum choices and school resources. This makes it a ripe target for government policies aimed at improving educational outcomes for students from low-SES families, compared with approaches targeting the influences of family environment or peer groups.