The Ready-Made Garments (RMG) industry in Bangladesh is highly dependent on operational efficiency, product quality and on-time delivery to maintain competitiveness in the global apparel market. While many studies focus on holistic Quality Management Systems (QMS), an empirical gap exists in quantifying the isolated impact of basic Standard Operating Procedure (SOP) enforcement on core production metrics. This study investigates the effect of SOP implementation - without integrating the Plan-Do-Check-Act (PDCA) cycle or other external quality models - on performance within the sewing and quality sections of a leading Bangladeshi RMG factory. A pre- and post-implementation analysis was conducted using key performance indicators (KPIs) such as efficiency, Non-Production Time (NPT), Handover on Time (HOT), defect rates and Defects per Hundred Units (DHU). Findings reveal a significant, immediate step-change in operational performance, demonstrating the technical contribution of process discipline. Production efficiency increased from approximately 67.9% to 77.9%, HOT achieved 100% on-time delivery, NPT reduced substantially (from 32% to 22.05%) and DHU rates dropped sharply (from 9.96% to 5.21%) due to standardized workflows and multi-stage quality checkpoints. These empirical results provide clear evidence that SOPs are a powerful, standalone lever for operational excellence.