This paper reports findings from a study that followed K-12 mathematics teachers through the components of a blended professional development program, highlighting their engagement in a series of online modules that connect face-to-face gatherings across the school year. Data collected over four years illuminate the teachers' performance, perceptions, and practice related to participation in blended professional learning. Results indicate that the blended learning experience led teachers to increased knowledge of mathematics content, greater use of high-quality resources, deeper knowledge of standards, and professionally rewarding interactions with other teachers. The findings support the use of blended learning approaches in K-12 mathematics professional development, leading to recommendations for practice and suggestions for future research.