Guidelines for the nutritional management of critically ill patients recommend the use of injectable lipid emulsion (ILE) as part of parenteral nutrition (PN). The ILE’s impact on outcomes remains unclear. Associations between prescribed ILE and in-hospital mortality, hospital readmission, and hospital length of stay (LOS) in critically ill patients in the intensive care unit (ICU) were investigated. Patients who were ≥18 years old in an ICU from January 2010 through June 2020, receiving mechanical ventilation, and fasting for >7 days, were selected from a Japanese medical claims database and divided, based on prescribed ILE during days from 4 to 7 of ICU admission, into 2 groups, no-lipid and with-lipid. Associations between the with-lipid group and in-hospital mortality, hospital readmission, and hospital LOS were evaluated relative to the no-lipid group. Regression analyses and the Cox proportional hazards model were used to calculate the odds ratios (OR) and regression coefficients, and hazard ratios (HR) were adjusted for patient characteristics and parenteral energy and amino acid doses. A total of 20,773 patients were evaluated. Adjusted OR and HR (95% confidence interval) for in-hospital mortality were 0.66 (0.62–0.71) and 0.68 (0.64–0.72), respectively, for the with-lipid group relative to the no-lipid group. No significant differences between the two groups were observed for hospital readmission or hospital LOS. The use of ILE for days 4 to 7 in PN prescribed for critically ill patients, who were in an ICU receiving mechanical ventilation and fasting for more than 7 days, was associated with a significant reduction in in-hospital mortality.