Background: Accurate staging of the mediastinal lymph nodes is of great importance to determine optimal treatment options in non-small cell lung cancer (NSCLC). In case of suspected mediastinal metastases, endoscopic/endobronchial ultrasound combined with mediastinoscopy is the gold standard. The diagnostic value of these procedures stands or falls by how they are technically performed. This study used data from the Dutch Lung Cancer Audit for Surgery to evaluate surgical performance of mediastinoscopies in The Netherlands. Methods: The study included all patients with a mediastinoscopy for staging of NSCLC and subsequent resection from 2012 to 2016. Complete case analysis was performed, excluding patients with missing data on biopsies or tumor side. Location and number of biopsied stations and adherence to guidelines for performing mediastinoscopy were analyzed. The proportion of unforeseen mediastinal lymph node metastases (unforeseen N2) was compared between mediastinoscopies that did or did not comply with the Dutch guideline. Results: The analysis included 1,729 patients. Mediastinoscopies were performed according to the Dutch guideline (requirements: biopsies of 2 ipsilateral stations, 1 contralateral station, and N7) in 51.4% (n = 888) and according to the European Society of Thoracic Surgeons guideline (N4 left, N4 right, and N7) in 75.4% (n = 1,303). Overall, unforeseen N2 was present in 10.2% (n = 140). In mediastinoscopies performed according to the Dutch guideline, unforeseen N2 occurred less often (8.6%) than in the nonadherence group (11.9%; p = 0.043). Conclusions: There is improvement potential in surgical performance of mediastinoscopy in The Netherlands, which is reflected by the percentage of guideline adherence and the occurrence of unforeseen N2.