Background It is important to characterize in-hospital mortality after cardiac surgery and understand the relationships between postoperative length of intensive care unit stay, postoperative length of hospital stay, and the likelihood of in-hospital mortality. Methods We retrospectively identified all cardiac surgery cases that resulted in in-hospital mortality over an 8-year period at a single center. For these subjects we collected demographic data, preoperative comorbidities, and postoperative complications. We performed stepwise multivariate linear regression to determine which postoperative complications were associated with mortality timing. We also analyzed the relationships between postoperative length of intensive care unit stay, postoperative length of hospital stay, and in-hospital mortality in all patients (including survivors) who had cardiac surgery during the same time period. Finally, we calculated the daily incremental observed mortality rate for patients in the hospital up to postoperative day 50. Results Six hundred twenty-one in-hospital mortalities occurred among 18,348 patients during the study period (3.4%). Four postoperative complications were associated with mortality timing. Cardiac arrest had a negative association with the number of days until mortality, while deep sternal wound infection, stroke, and pneumonia had a positive association (all p < 0.05). Postoperative complications explained 15% of the variability in mortality timing (R2 model = 0.15). The odds ratio for in-hospital mortality was 1.033 for each postoperative day in the hospital and 1.071 for each postoperative day in the intensive care unit (both p < 0.05). Conclusions Most in-hospital mortality occurs during the first week after cardiac surgery with few mortalities occurring after a protracted hospital course. Postoperative complications have a limited ability to explain the variability in mortality timing. Increased length of postoperative intensive care unit stay and hospital stay after cardiac surgery are associated with an increased likelihood of in-hospital mortality.