Background <p>Intercalary allograft after resection of bone tumors is associated with low failure rates. However, literature on the long-term outcomes of this technique is limited, and long-term functional outcomes remain unknown. This study sought to assess allograft survival, risk factors for failure, and functional outcomes for patients with a minimum 10-year follow-up period.</p> Methods <p>This study retrospectively identified patients treated with intercalary allograft of the femur or tibia for oncologic indications. Allograft failure was assessed using the Henderson classification for biologic reconstruction. Early, midterm, and late failures were defined as those occurring before 2 years, between 2 and 10 years, and after 10 years of follow-up evaluation, respectively.</p> Results <p>Of the 64 patients in the study, 29 had a femur allograft and 35 had a tibia allograft. The median follow-up period was 22 years. Complications developed in 34 patients (53%), with 79% having multiple complications. Non-union failures (type 2) comprised 42% of the early failures and 43% of the midterm failures, whereas structural failures (type 3) represented 100% of the late failures. The risk factors for allograft failure were neoadjuvant radiotherapy and chemotherapy. The median TESS-LE and LEFS scores were 97.3% and 76.3% for the patients with femur allografts and 98.3% and 86.3% for those with tibia allografts, respectively. The PROMIS Pain Interference and Physical Function scores were 51.2 and 45.2 for the patients with femur allografts and 40.7 and 50 for those with tibia allografts, respectively.</p> Conclusion <p>Non-union and structural failures were the most common causes of early and late failure, respectively. Although complications occurred frequently, the long-term functional outcomes for femur and tibia intercalary allografts were good.</p>