Apparel manufacturers face persistent challenges in maintaining product quality, improving production efficiency, and meeting increasingly demanding delivery schedules within volatile global supply chain networks. Recurring sewing defects and inconsistent shop-floor practices highlight the need for systematic continuous improvement (Kaizen) approaches that address root causes and sustain operations performance. Because Toyota’s 8-Step Problem-Solving Method is based on the 4 step PDCA (Plan-Do-Check-Act) cycle, it provides an actionable framework to institutionalize lean routines and eliminate process waste. This study evaluates the implementation of this methodology and its implications for defect rates, sewing-line efficiency, manufacturing lead time, and standardized problem-solving routines. A longitudinal single-case study was conducted at an apparel manufacturing enterprise in West Java, Indonesia, focusing on six high-volume sewing lines over 12 months. Operations data from baseline and post-implementation phases were analyzed descriptively, while paired t-tests were used to assess performance changes. The results showed that the average defect rate decreased from 8.42% to 2.15%, sewing line efficiency increased from 58.30% to 81.60%, and manufacturing lead time declined from 24.50 to 14.20 days. Daily output per line also increased by 39.92%. All three primary performance indicators changed significantly (p < 0.001). The findings indicate that structured problem-solving, supported by root-cause analysis, targeted countermeasures, monitoring, and standardization, directly strengthens operational capability and downstream delivery reliability across the supply chain. However, further research is needed to establish the long-term sustainability and generalizability of these improvements across different industrial contexts.