Corporate Environmental Sustainability (CES) in developing economies remains under explored, particularly in how institutional pressures influence environmental practices in large manufacturing firms. This study examined how institutional pressures shape CES among large manufacturing firms in a developing economy context. A qualitative, inductive approach grounded in phenomenology was applied. Data were collected through 24 in-depth interviews with firm-level participants and regulatory officials and analyzed using reflexive thematic analysis supported by NVivo. The study identified that coercive (regulatory compliance, audits), normative (professional standards, environmental expectations), and mimetic (peer learning, brand reputation) pressures collectively shape environmental behavior through distinct isomorphic mechanisms. The study proposed a novel framework contextualized within developing economies, offering actionable insights for policy, regulation, and industry practice, while offering a foundation for future empirical testing of institutional and isomorphic dimensions in CES.