Transforming mathematics learning and teaching toward more equitable and effective approaches is critical to student mathematics learning and identity development. This task at-scale in a district takes time, commitment, and mathematics expertise that may not be widespread in the absence of focused professional development. District and regional mathematics leaders with university mathematics and mathematics education faculty partnered to address this challenge by designing a professional development program that prepared K-12 teachers of mathematics as leaders at the classroom, school, and district level. Described are specific aspects of a professional learning model focused on developing mathematics content, pedagogical, and leadership knowledge and skills. Also provided are findings related to the impact of the project's professional development on shifts in instructional practice and student achievement.