Background This study aimed to assess factors that affect union time and complications in Gustilo IIIC tibial fractures. Methods This retrospective analysis included patients with IIIC open tibial fractures who visited our center from January 2000 to October 2020. Patient demographics, fracture characteristics, timing, numbers, and type of surgical intervention were documented. Outcomes included union time, occurrence of osteomyelitis, and amputation. Results Fifty-eight patients were enrolled and grouped by fracture type: eight as union on time (13.8%); 27 late union (46.6%); eight delayed union (13.8%); three nonunion (5.2%); and 12 amputations (20.7%). Nine fractures (15.5%) were complicated by osteomyelitis. Union time was prolonged in cases of triple arterial injury, distal third fractures, multiple trauma with Injury Severity Score ≥ 16, and increased length of bone defect. A bone gap > 50 mm, diabetes mellitus, low body mass index, and triple arterial injury in the lower leg were significant risk factors for amputation. Time from injury to definitive soft tissue coverage that was longer than 22 days was the major risk factor for osteomyelitis. The predicted probability of union within 2 years was stratified based on a scoring system predicting union time. Conclusions Gustilo IIIC tibial fractures involving the distal third of the tibia, fractures with bone defects, triple arterial injury, and multiple trauma with ISS ≥ 16 demonstrated delayed union, and an effective system predicting union time was built. Early soft tissue coverage reduces the risk of osteomyelitis, while diabetes and severe bone and soft tissue defects increase the risk of amputation.