This study aimed to compare the incidence and severity of cervical kyphosis before and after surgery between patients with adolescent idiopathic scoliosis (AIS) with major thoracolumbar/lumbar curves (Lenke type 5C group) and those with major thoracic curves (Lenke type 1A group). Furthermore, factors associated with postoperative changes in cervical spinal alignment were examined in both groups. We included consecutive patients with AIS who underwent posterior spinal fusion for Lenke type 1A and 5C curves with a minimum one-year follow-up. To assess sagittal alignment, radiographs were evaluated preoperatively, immediately postoperatively, and at one year after surgery. Correlations between the change in the C2–C7 angle (∆C2–C7) and other spinopelvic parameters were analyzed. Preoperatively, cervical kyphosis was present in 19 of 30 patients in the Lenke 1A group and 21 of 36 in the Lenke 5C group. The incidence of cervical kyphosis did not significantly differ between the two groups. Postoperatively, both groups demonstrated significantly increased thoracic kyphosis (TK) and larger C2–C7 angles. In the Lenke 5C group, TK further increased at one year postoperatively. While the C2–C7 angle was maintained in the Lenke 5C group, the Lenke 1A group showed a significant re-decrease in the C2–C7 angle at the one-year follow-up. Analysis revealed that ∆TK was closely associated with ∆C2–C7 in the Lenke 1A group, but not in the Lenke 5C group. These findings suggest that in thoracic AIS, postoperative cervical alignment should be managed by achieving adequate TK and promoting coronal plane correction. Moreover, selective corrective surgery can effectively improve postoperative cervical alignment in lumbar AIS.