This phenomenological study examined how Social Studies teachers adapted their instruction in public high school mainstream classrooms. Guided by Universal Design for Learning, Differentiated Instruction, and Teacher Self-Efficacy, the study explored the experiences that emerged as teachers adapted instructional practices for learners in mainstream classrooms, the adaptive mechanisms they employed, and the support mechanisms that could be provided to Social Studies teachers handling mainstream classes. Six Social Studies teachers experienced in handling learners with diverse learning needs were selected through purposive sampling. Data were gathered through a researcher-made semi-structured interview guide and analyzed using Moustakas’ transcendental phenomenological procedures. Findings were organized into three thematic areas: Experiences in Instructional Adaptation, Inclusive Adaptive Mechanisms, and Support Mechanisms Needed by Social Studies Teachers. Results showed that teachers made Social Studies lessons more accessible through localized examples, visual aids, simplified materials, peer support, flexible outputs, and modified assessment conditions. However, participants emphasized that inclusive education becomes difficult when mainstream placement is not supported by training, instructional materials, manageable classroom conditions, and specialist assistance. The study concludes instructional adaptation in Social Studies is a continuous, reflective, and context-based situated process.