Social and emotional development in early childhood has been shown to predict long-term outcomes, including school readiness and academic achievement. Early years practitioners need robust assessment tools to adequately identify early signs of social, emotional, behavioural and mental health (SEMH) difficulties. The current study aimed to develop and validate the Early Years Boxall Profile (EYBP). A total of 510 children were assessed by preschool practitioners using the EYBP and the Strengths and Difficulties Questionnaire. We found that the EYBP showed limited accuracy in screening children with SEMH difficulties. However, the EYBP had good internal consistency and excellent convergent and concurrent validity. Results suggest that the tool may be helpful to identify early signs of SEMH needs. Future research is needed to explore the structural validity of the assessment tool and to improve its diagnostic abilities.