Identification of likelihood for autism in the U.S. early intervention (EI) system facilitates referrals to diagnostic assessments and targets autism-specific needs. However, performance of autism screeners in high-likelihood preschool children and barriers to implementation need to be further studied. The current effectiveness-implementation hybrid type 1 study examined the impact of administering the Modified Checklist for Autism in Toddlers, Revised with Follow-Up (M-CHAT-R/F) in a sample of preschool children during the EI intake phone call. Data from 2,661 educational records showed that 42.8% of them received the M-CHAT-R. Only 14.0% of the screen-positive children completing the process to determine eligibility for EI services were referred for an autism-specific evaluation. Most of these (94.8%), regardless of screener status, received an autism diagnosis, suggesting that referrals are based on clinical judgment. Training and incorporation to current protocols within agencies are necessary to introduce a screening tool and support implementation fidelity and effective autism detection.