Background: The inflammatory response plays a central role in the clinical outcomes of cerebrovascular disease. The aim of this study was to investigate the clinical significance of pan-immune-inflammation value (PIV) in patients with acute ischemic stroke after mechanical thrombectomy (MT). Methods: The study included 201 patients who underwent MT. Blood samples taken from the patients before the procedure were evaluated and inflammation markers were calculated. Severity of stroke was assessed using the National Institute of Health Stroke Scale (NIHSS) scores on admission. Poor 3-month functional outcome was defined as Modified Rankin Scale (mRS) scores of >2. Ischemic stroke types were classified according to the Trial of Org 10172 in Acute Stroke Treatment (TOAST) classification. Results: In the logistic regression analysis, we observed that PIV was associated with a poor outcome. Post hoc multiple comparison tests revealed statistically significant differences in PIV between the stroke of other determined etiology and small-vessel occlusion (178.00 vs. 74.89, p = 0.015 and p < 0.05, respectively), large artery atherosclerosis (178.00 vs. 95.51, p = 0.032 and p < 0.05, respectively), and cardioembolism (178.00 vs.107.97, p = 0.043 and p < 0.05) subtypes. There was a moderate positive statistically significant relationship at the 95% confidence level between NIHSS score and PIV (r = 0.696, p < 0.05). Conclusion: Our study revealed that PIV predicts a poor 3-month prognosis in acute ischemic cerebrovascular disease after MT with a significantly better performance than the widely known systemic immune-inflammation index, systemic inflammation response index, platelet/lymphocyte ratio, and neutrophil/lymphocyte ratio. PIV can be a novel prognostic marker indicating poor prognosis in patients treated with MT.