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Background High-risk cardiac surgery is commonly complicated by low cardiac output syndrome (LCOS) which is associated with high mortality. There are limited data derived from multi-center studies with adjudicated endpoints describing factors associated with LCOS and its downstream clinical outcomes. Methods The Levosimendan in Patients with Left Ventricular Systolic Dysfunction Undergoing Cardiac Surgery Requiring Cardiopulmonary Bypass (LEVO-CTS) trial evaluated prophylactic levosimendan versus placebo in patients with a reduced ejection fraction undergoing coronary artery bypass grafting and/or valve surgery. We conducted a pre-specified analysis on LCOS which was characterized by a four-part definition. We constructed a multivariable logistical regression model to evaluate risk factors associated with LCOS and performed Cox proportional hazards modeling to determine the association of LCOS with 90-day mortality. Results A total of 186 (22%) of 849 patients in the LEVO-CTS trial developed LCOS. Factors most associated with a higher adjusted risk of LCOS were pre-operative ejection fraction (OR 1.26; 95% CI: 1.08–1.46 per 5% decrease) and age (OR 1.13; 95% CI: 1.04–1.24 per 5-year increase) whereas isolated CABG surgery (OR 0.44, 95% CI: 0.31–0.64) and levosimendan use (OR 0.65; 95% CI: 0.46–0.92) were associated with a lower risk of LCOS. Patients with LCOS had worse outcomes, including renal replacement therapy at 30-days (10% vs. 1%) and 90-day mortality (16% vs. 3%, adjusted HR of 5.04, 95% CI: 2.66–9.55). Conclusions LCOS is associated with a high risk of post-operative mortality in high risk cardiac surgery.