Many training initiatives are underway to increase implementation of evidence-based practice (EBPs) in mental healthcare. However, little is known about what types of trainings and supports yield the highest reach and engagement. Supported by a tax-funded, countywide initiative to improve access to quality care for youths, the current mixed methods study evaluates mental health (MH) provider reach, or registering for the training initiative, and engagement, or participation in training activities, for several EBP training and implementation supports. MH providers were offered free 1) formal EBP workshops, 2) a biweekly learning community, 3) individual case consultation, and 4) confidential online clinical feedback system. To register, interested providers (N = 698) completed a web-based assessment measuring clinical practice information, organizational implementation climate, and EBP knowledge, attitudes, and practices. Thirteen providers, selected via purposeful sampling stratified by level of participation, completed semi-structured qualitative interviews. While the training initiative achieved high reach (66% of county agencies had a provider register), far fewer providers engaged substantially in training. Quantitative results indicated that providers whose professional discipline was not psychology, had higher baseline EBP knowledge, more extensive use of common evidence-based strategies, and less extensive use of other therapy strategies, engaged in more training. Rapid qualitative analysis of interviews expanded upon these findings and illuminated provider, organizational, system, practical, and training activity-specific barriers and facilitators to engagement. Findings suggest the importance of identifying strategies for improving provider engagement in training activities beyond workshops. Implications for future research and training initiatives are discussed.