Background <p>Siewert type II adenocarcinoma of the esophagogastric junction (AEG) is characterized by complex, bidirectional lymph node metastasis. This retrospective study evaluates recurrence and mortality risks associated with varying extents of mediastinal lymph node dissection.</p> Methods <p>A retrospective analysis was conducted on 221 Siewert type II AEG patients who underwent radical surgery (McKeown or Left Thoracotomy [LT] approach) between June 2016 and October 2023. Patients were grouped according to the extent of lymphadenectomy: complete mediastinal and abdominal lymph node dissection (CMAD) or middle/lower mediastinal and abdominal lymph node dissection (MLMAD). To ensure comparability, patients in the CMAD group were matched 1:1 with those in the MLMAD group using propensity score matching (PSM). Prognostic outcomes were analyzed using Kaplan–Meier curves, and Cox regression analysis was performed to identify risk factors.</p> Results <p>CMAD was associated with significantly improved overall survival (OS) (HR = 0.37, 95%CI:0.23–0.60; <i>P</i> &lt; 0.001) and disease-free survival (DFS) (HR = 0.36, 95%CI:0.22–0.58; <i>P</i> &lt; 0.001) compared to MLMAD before PSM; these benefits persisted after PSM (OS HR = 0.44, <i>P</i> = 0.01; DFS HR = 0.44, <i>P</i> = 0.02). Upper mediastinal metastasis was observed in 4.3% of patients. Dissection of upper mediastinal stations, particularly 2R, 4R, and 8U, contributed to the survival benefit. Removal of more than 11 lymph nodes was associated with improved survival. The McKeown approach enabled a more comprehensive upper mediastinal and abdominal lymphadenectomy compared to LT (<i>P</i> &lt; 0.001), without increasing complication rates or hospital stay.</p> Conclusion <p>The findings suggest that a surgical strategy involving the comprehensive removal of mediastinal and abdominal lymph nodes, when applied to selected patients in a high-volume center, is associated with superior survival outcomes compared to more limited dissections.</p>