Background: On 1 October 2019, the Taiwan National Health Insurance (NHI) Administration introduced reimbursement for nutritional care services provided to intensive care unit (ICU) patients, under the category of nutritional care fees. These services included the implementation of structured, dietitian-guided individualized nutrition (DGIN) protocols designed to address the clinical needs of critically ill patients. Objectives: This study aimed to evaluate the effectiveness of DGIN in critically ill patients following the implementation of NHI coverage. Methods: This retrospective cohort study was conducted in the ICU of a tertiary medical center, including patients admitted between 1 September 2018 and 31 October 2020, encompassing periods both before and after the initiation of NHI coverage. Under NHI coverage, dietitian-guided individualized nutritional support was provided within the first two weeks of admission. The intervention group received DGIN within the first two weeks of ICU admission. A total of 5292 patients were screened; 2381 were included in the final analysis (1116 in the standard care (SC) group and 1265 in the DGIN group), categorized based on the timing of NHI coverage and the corresponding frequency of dietitian visits. The DGIN protocol comprised a baseline assessment within 24–48 h and three structured reviews during the first ICU week, while the comparator received SC. Demographic characteristics, daily nutritional data, and clinical outcomes were analyzed. Results: Significant baseline differences in nutritional intake and disease severity were observed. Following the introduction of the DGIN protocol, the intervention group received more structured and closely monitored nutrition management, which resulted in less aggressive caloric intake. This approach was associated with a significantly shorter ICU length of stay (SC: 8.1 ± 6.7 days vs. DGIN: 7.1 ± 7.4 days, p < 0.001). Conclusions: An ICU nutritional care plan involving frequent assessments and timely interventions by clinical dietitians is associated with a reduced ICU length of stay in critically ill patients. These findings support the effectiveness of integrating dietitian-led nutritional care into national health insurance coverage for ICU patients.