The number of English learners (ELs) entering middle schools continues to increase, and middle school content teachers are providing EL instruction face a considerable challenge. The problem studied in this research was the middle school content teachers’ insufficient understanding of ELs’ cultural background, needs, and interests. The literature review indicated middle school content teachers receive training or workshops based on the assigned subject taught and less on accommodating ELs. This qualitative case study aimed to explore middle school mainstream teachers’ preparations, perceptions, and experiences encountered while teaching ELs in one Maryland school district. Vygotsky’s (1978) sociocultural theory and the concept of the zone of proximal development framed the study. The research questions used to clarify the problem focused on the factors affecting ELs’ education, such as teachers’ preparation, perceptions, and experiences while teaching ELs in the mainstream classroom. Fifteen middle school content teachers took part in the study and answered the online questionnaire and interview questions in detail with interviews conducted one-on-one. Criterion sampling was used to identify the 15 teacher volunteers who completed online teacher questionnaires and one-on-one interviews. The data from two research instruments were cleaned, prepared, and analyzed, resulting in emergent themes. Based on the findings, there is a need for relevant professional development and in-service training for content teachers teaching ELs in the mainstream classroom. The need for greater collaboration among all stakeholders was suggested to support content teachers in more effectively instructing ELs.