The primary goal of the Intensified Algebra I (IA) program is to enable mathematically underprepared students to successfully complete Algebra I in 9th grade and stay on track to meet increasingly rigorous high school mathematics graduation requirements. The program was designed to bring a range of both cognitive and non-cognitive supports to bear on underprepared students' learning of rigorous algebra content within an extended period of instruction, thus allowing them to catch up with their peers on a pathway toward more advanced mathematics courses. This study measured gains in IA students' overall mathematical performance based on a comprehensive multiple-choice assessment of algebra proficiency and a constructed response assessment. Results showed IA students' performance significantly increased over the academic year on both assessments. In addition, students' performance showed a consistently large improvement in three of six core content areas (Graphing Linear Equations, Functions and Graphs, and Solving Linear Equations) within the multiple-choice assessment. This study provides promising evidence of IA meeting its programmatic goal of supporting underprepared students learning of core algebra content in a function-based curriculum. Implications for curriculum implementation and ongoing development, and further research are discussed.