Several studies have investigated the predictors of functional outcome in patients with ischemic stroke after mechanical thrombectomy (MT). However, it is not clear whether pre-stroke cognitive (PSC) impairment is associated with the functional outcome of patients treated with MT. We enrolled 113 patients treated with MT from December 2016 to November 2018. PSC was evaluated using the Informant Questionnaire on Cognitive Decline in the Elderly (IQCODE). Poor outcome was defined as a modified Rankin Scale score of 3-6. We compared the clinical characteristics between the groups with poor outcome (n = 61) and good outcome (n = 52) to determine if PSC could be a predictor of poor outcome. IQCODE was significantly higher in the group with poor outcome than good outcome (3.34 vs. 3.13, P = 0.017). Moreover, the following metrics differed between those two groups: age (75.9 vs. 71.6 years old, P = 0.010), the percentage of females (39.9% vs. 17.3%, P = 0.009), the percentage with hypertension (72.1% vs. 44.2%, P = 0.003), National Institutes of Health Stroke Scale (NIHSS) score on admission (20 vs. 11, P < 0.001), and no successful recanalization (24.5% vs. 7.7%; P = 0.025). Multivariable logistic regression analysis demonstrated that PSC (OR: 5.59; 95% CI: 1.55-23.47), history of hypertension (OR: 3.33; 95% CI: 1.29-9.11), no successful recanalization (OR: 5.51; 95% CI: 1.49-25.03), and NIHSS score on admission (OR: 1.14; 95% CI: 1.07-1.22) were associated with poor outcome 3 months after stroke onset. PSC was significantly and independently associated with poor functional outcome in patients treated with MT.