Teaching systematic phonics effectively to beginning readers requires specialized knowledge and training which many primary grade teachers lack. The current study examined effects of a year-long mentoring program to improve teachers' knowledge and effectiveness in teaching phonics and the extent that it improved students' achievement in reading and spelling. Teachers in urban, lower SES schools completed a 45 h course followed by 90 h of in-school training. Mentors (N = 29) worked with kindergarten, 1st, 2nd, and 3rd grade teachers (N = 69) twice a week for 30 weeks during the year. Each visit included a 45 min prep period plus 45 min of modeling and feedback in the classroom. Mentors taught teachers how to provide systematic phonics instruction to their students (N = 1,336). Monthly ratings by mentors revealed that teachers improved their phonics teaching skills with many reaching the highest ratings by May. Teachers who were non-native speakers of English took a bit longer to learn the English sound system for letters, mainly because they lacked sufficient knowledge of English sounds and had to learn them. Given the increasing diversity of the teacher work force, future research is needed to study this difficulty, its solution, and impact on students. Teachers' agreement with principles of phonics instruction remained strong or increased from fall to spring. Students' reading and spelling skills showed large gains during the year and far exceeded effect sizes from comparable data sources. Students met grade-level expectations at the end of kindergarten and first grade but fell short in second and third grades. General education students outperformed bilingual/ELL and special needs students although all subgroups made large gains. Findings reveal the effectiveness of an intensive mentoring model of professional development applied to a subject that is difficult to teach and to a student population known for lower reading achievement. Findings point to the need for better pre-service teacher preparation coupled with appropriate curricula and PD from districts in order to improve students' reading achievement.