Background: Mechanical thrombectomy (MT) has become the standard of care for acute ischemic stroke with large vessel occlusion; however, evidence remains insufficient for MT for elderly patients, especially with respect to factors affecting their outcomes. Methods: This study was a retrospective analysis of a multicenter registry of MT, called Tama Registry of Acute Endovascular Thrombectomy. Patients were divided by their age into 2 groups: Nonelderly (NE; < 80) and elderly (E; ≥80). Factors related to a good outcome (modified Rankin scale score ≤2) were examined in each group. Onset to reperfusion time (OTR) was stratified into 4 categories: category 1, 0 – ≤180 min; category 2, > 180 – ≤360 min; category 3, > 360 min or onset time not identified; and category 4, effective recanalization not achievable. Results: 143 NE patients and 78 E patients were included in this study. The E group had less chance of achieving a good outcome (NE group 51%, E group 35%; p = 0.024). In the NE group, lower OTR category was an independent prognostic factor for good outcome (p = 0.037, OR = 1.09). However, in the E group, OTR category was not a significant predictor on multivariate analysis. Instead, effective recanalization (p = 0.0081, OR 1.40) and lower National Institute of Health Stroke Scale score at presentation (p = 0.0032, OR 1.02) were the independent predictors. Conclusions: In MT for elderly patients, effective recanalization improved the patients’ outcome but OTR affected less. Further studies are warranted to establish the appropriate patient selection and treatment strategies.