Purpose Despite significant interest in improving behavioral health therapists' implementation of measurement-based care (MBC)-and widespread acknowledgment of the potential importance of organization-level determinants-little is known about the extent to which therapists' use of, and attitudes toward, MBC vary across and within provider organizations or the multilevel factors that predict this variation. Methods Data were collected from 177 therapists delivering psychotherapy to youth in 21 specialty outpatient clinics in the USA. Primary outcomes were use of MBC for progress monitoring and treatment modification, measured by the nationally-normed Current Assessment of Practice Evaluation-Revised. Secondary outcomes were therapist attitudes towards MBC. Linear multilevel regression models tested the association of theory-informed clinic and therapist characteristics with these outcomes. Results Use of MBC varied significantly across clinics, with means on progress monitoring ranging from values at the 25th to 93rd percentiles and means on treatment modification ranging from the 18th to 71st percentiles. At the clinic level, the most robust predictor of both outcomes was clinic climate for evidence-based practice implementation; at the therapist level, the most robust predictors were: attitudes regarding practicality, exposure to MBC in graduate training, and prior experience with MBC. Attitudes were most consistently related to clinic climate for evidence-based practice implementation, exposure to MBC in graduate training, and prior experience with MBC. Conclusions There is important variation in therapists' attitudes toward and use of MBC across clinics. Implementation strategies that target clinic climate for evidence-based practice implementation, graduate training, and practicality may enhance MBC implementation in behavioral health.