Although children residing in statutory out-of-home care and those adopted from care are more likely than not to have mental health difficulties requiring clinical intervention or support, their difficulties often remain undetected. Children’s agencies have a duty of care to identify those child clients who require therapeutic and other support services, without regard to the availability of such services. The present article proposes a first-stage mental health screening procedure (calibrated for high sensitivity) for children and adolescents (ages 4–17) in alternative care, which children’s agencies can implement without clinical oversight using the Strengths and Difficulties Questionnaire (SDQ) and Brief Assessment Checklists (BAC). The screening procedure was derived from analyses of BAC, SDQ, and “proxy SDQ” scores obtained in three national studies of children and adolescents residing in alternative care (Australia, the Netherlands, and England). The SDQ and BAC demonstrated moderate to high screening accuracy across a range of clinical case criteria—the SDQ being slightly better at predicting general mental health problems and the BAC slightly better at predicting attachment- and trauma-related problems. Accurate first-stage screening is achieved using either the SDQ or the BAC alone, with recommended cut points of 10 (i.e., positive screen is 10 or higher) for the SDQ and 7 for the BAC. Greater accuracy is gained from using the SDQ and BAC in parallel, with positive screens defined by an SDQ score of 11 or higher or a BAC score of 8 or higher. Agencies and post-adoption support services should refer positive screens for comprehensive mental health assessment by clinical services.