Background: The importance of professional development training for individuals tasked with providing quality early child care is widely accepted. However, research assessing the impact of specific, long-term professional development programs on changes in caregiver behavior is largely absent, as is research about the processes and mechanisms of such training efforts that produce changes in child care practices. Objective: We specify the underlying activities and processes of a mentor-delivered quality enhancement and professional development program, Family Child Care Partnerships (FCCP), and present the results of two studies using implementation data to examine program effectiveness as measured by mentors' observational assessments of the global quality of caregiving practices and providers' self-reported professional engagement. Methods: Study 1 consisted of 365 family child care providers who were invited to participate in FCCP. Study 2 was a subsample of Study 1 and consisted of 109 providers. We used latent difference score modeling to examine changes in global child care quality from program entry to program departure and bivariate associations among changes in quality and professional engagement. Results: Significant increases were found between pre- and post-enrollment assessments of caregiving quality and professional engagement. Increases in professional engagement were associated with increases in quality. Conclusions: Findings are discussed in the context of the limited information available to guide quality improvement and professional development in family child care and in light of recommendations that the early childhood care and education field develop methods of training that embed knowledge and skills development in practice.