<p>To evaluate Reach, Effectiveness, Adoption, Implementation, and Maintenance (RE-AIM) of First Step, a brief intervention, delivered as the first stage in care for adults seeking community alcohol and other drug (AOD) treatment. A single-arm implementation effectiveness trial was conducted. Participants were clients and staff of three community outpatient services. Administrative data (e.g., number of enquiries, enrolments and treatment sessions before, during and after First Step implementation, First Step module commencement and completion of core components), client outcomes data at 1-, 3-, 6, and 12-months follow-up (e.g., severity, quantity, and frequency of AOD; mental health) and counsellor data at 6-, 12- and 18-months post-training (e.g., intentions and confidence delivering First Step) were used. Enrolled clients attended a median of 2–3 treatment sessions (range 0–41). Compared to before the First Step trial, there was an increase in the proportion of clients who enrolled and received treatment. Data from 418 clients (83% of enrolments; 58.8% male, mean age = 36.4 years) revealed significant improvements in client AOD severity and mental health outcomes over 12-months with small-moderate sized effects. There was wide variability in the adoption of First Step by counsellors. Most clients received content from at least one First Step module during (<i>n</i> = 346, 83%) and after (<i>n</i> = 598, 80%) the First Step trial, but the completion rates were low (28% and 18%). Poor agreement was found between independent and counsellor ratings of the completion of core First Step components, suggesting counsellor ratings had low reliability. Future research using continual monitoring of First Step delivery with corrective feedback and supervision is needed to determine how to best implement First Step and whether it produces better results than treatment as usual.</p>