It is established that the development of psychological well-being and cognitive skills for Looked After and Adopted Children/Young People (LAACYP) is impacted by their experiences before and after entry into care. Language and communication skills are typically considered to have a reciprocal relationship with cognitive development, but the language skills of the LAACYP population are currently underspecified. The current study explores the severity and specificity of language and communication difficulty in a LAACYP population and the association between these difficulties and several care-related factors, including mental health and Adverse Childhood Experiences (ACEs). Fifty-seven children aged 4–16 either in foster care or an adoptive placement were sampled via carer-report, using the Children’s Communication Checklist 2. Forty-two of these children’s mental health needs were also sampled using the Assessment Checklist for Children. The child’s social worker completed information regarding ACEs and care-related factors. Based on UK norms, the performance of 68% of the children sampled was consistent with the lowest 10% of the general population for language and communication ability. Similar levels of severity were found for both structural and pragmatic abilities. Language and communication difficulties were associated with older children, later age of entry into care and placement type (foster care vs. adoption) but not placement disruption. A significant relationship was found between mental health and language difficulty, but no link was established with ACEs. This study highlights the severity and pervasiveness of language and communication impairment among many of the LAACYP population and its association with psychological well-being.