EFL instruction in higher education calls for curriculum designs that draw on co-curricular engagement to support sustainability and student development. So, tertiary institutions are expected to correspond with it. Nonetheless, the pedagogical possibilities of co-curricular engagement are not fully utilized in the formal curricula of English as a Foreign Language (EFL) programs. This paper investigates how principles based on co-curricular involvement can guide sustainable curriculum development and teaching in higher education EFL settings. The proposed study is based on transformative learning and sustainability-oriented education frameworks. It follows a mixed-method design, incorporating curriculum and syllabus analysis, classroom observations, surveys among undergraduate students, and semi-structured interviews with EFL teachers. The results reveal that curriculum designs based on co-curricular principles increase the motivation, communication competence, and critical awareness of learners because they place the art of language within meaningful learning and social contexts. Educators and learners noted a change in the nature of instruction to inquiry-based and participatory teaching and learning, respectively. However, strict assessment systems and insufficient professional support remain as barriers to systematic integration. This study states that incorporating co-curricular involvement into EFL curricula enhances sustainability and instructional relevance without undermining linguistic rigor, which has practical implications for curriculum designers, educators, and policymakers.