Student engagement in their own learning of mathematics, and student attitudes towards mathematics are key dimensions of learning. This mixed methods study examined the impact of a classroom intervention based on the principles of reform mathematics (i.e., active student involvement, hands-on, real-world connections, use of manipulatives and technology, extensive use of student groups), on student engagement and student attitudes, in three Grade 10 mathematics classrooms in Ontario, Canada. Statistically significant effects were found for both the students' engagement and attitudes. The implications for pedagogy and policy indicated that the efficacy of reform mathematics principles influenced the students' affective dimensions in mathematics learning.