The advancement of deformity-specific implants and surgical techniques has improved the surgical treatment of Adult Spine Deformity (ASD), allowing surgeons to treat more complex deformities. Simultaneously, high rates of medical and surgical complications have been reported. The aim of this study is to describe the risk factors, the rate and the clinical impact of mechanical complications in ASD surgery. A retrospective review of a large, single-center database of consecutive ASD patients was conducted. Inclusion criteria were as follows: Cobb coronal curve > 20° or alteration of at least one of sagittal vertical axis (SVA > 40 mm), thoracic kyphosis (TK > 60°), pelvic tilt (PT > 20°) and pelvic incidence minus lumbar lordosis mismatch (PI-LL > 10°), at least four levels of posterior instrumented fusion and 2-year follow-up. At the baseline and at each follow-up end point, the authors collected clinical and radiographic outcomes and recorded any mechanical complications that occurred. One hundred and two patients were enrolled. Clinical outcomes significantly were improved at the last follow-up (mean 40.9 months). Postoperative mechanical complications occurred in 15 patients (14.7%); proximal junctional disease was the most common complication (60%) and the revision rate was 53.3%. Patients who experienced mechanical complications were older (61.2 vs. 54.8 years, p = 0.04); they had also a higher rate of pelvic fusion and posterior-only approach, a lower LL (-37.9 vs. -46.2, p = 0.02) and a higher PT (26.3 vs. 19.8, p = 0.009), TK (41.8 vs. 35.7, p = 0.05), PI-LL (12.9 vs. 5.4, p = 0.03) and Global Alignment and Proportion score (6.9 vs. 4.3, p = 0.01). This study showed a significant improvement in pain and disability after ASD surgery. Regarding the risk of developing a mechanical complication, not only postoperative radiographic parameters affected the risk but also patient age and surgical features.