Purpose: Patients with reduced muscle mass have a worse outcome, but muscle mass is difficult to quantify in the ICU. Urinary creatinine excretion (UCE) reflects muscle mass, but has not been studied in critically ill patients. We evaluated the relation of baseline UCE with short-term and long-term mortality in patients admitted to our ICU. Methods: Patients who stayed ≥ 24 h in the ICU with UCE measured within 3 days of admission were included. We excluded patients who developed acute kidney injury stage 3 during the first week of ICU stay. As muscle mass is considerably higher in men than women, we used sex-stratified UCE quintiles. We assessed the relation of UCE with both in-hospital mortality and long-term mortality. Results: From 37,283 patients, 6151 patients with 11,198 UCE measurements were included. Mean UCE was 54% higher in males compared to females. In-hospital mortality was 17%, while at 5-year follow-up, 1299 (25%) patients had died. After adjustment for age, sex, estimated glomerular filtration rate, body mass index, reason for admission and disease severity, patients in the lowest UCE quintile had an increased in-hospital mortality compared to the patients in the highest UCE quintile (OR 2.56, 95% CI 1.96–3.34). For long-term mortality, the highest risk was also observed for patients in the lowest UCE quintile (HR 2.32, 95% CI 1.89–2.85), independent of confounders. Conclusions: In ICU patients without severe renal dysfunction, low urinary creatinine excretion is associated with short-term and long-term mortality, independent of age, sex, renal function and disease characteristics, underscoring the role of muscle mass as risk factor for mortality and UCE as relevant biomarker.