Malnutrition with hypoalbuminemia (albumin < 35 g/L) is an important factor in predicting risks associated with colorectal cancer surgery. However, there is limited data about the effects of mild hypoalbuminemia with small decreases in albumin on postoperative complications. This is a retrospective study using the multi-institutional, nationally validated database of the American College of Surgeons-National Surgical Quality Improvement Program (ACS-NSQIP) to investigate mild hypoalbuminemia and its association with postoperative mortality and morbidity by using a propensity score matching method. In a group of 30,676 colorectal cancer patients who received surgery, 5230 had mild hypoalbuminemia (< 35 and > =30 g/L) and 21,310 had normal albumin levels (> = 35 g/L). Significant differences were noted in 21 clinical characteristics between the two groups. After 1:2 propensity score matching postoperative mortality was significantly associated with mild hypoalbuminemia (OR = 1.74; p < 0.001). There were significant associations between mild hypoalbuminemia and 11 postoperative morbidities including deep vein thrombosis, pulmonary embolism, superficial and deep surgical site infection, pneumonia, septic shock, ventilator> 48 h, blood transfusion, return to operating room, stroke and re-intubation. Mild hypoalbuminemia was also associated with overall complication (B = 0.064, p < 0.001) and length of total hospital stay (B = 2.236, p < 0.001). In colorectal cancer, this is the first propensity score matching study of malnutrition with mild hypoalbuminemia which demonstrates that a mild decrease in serum albumin contributes significantly to poor postoperative outcome.