Background: Stroke patients ≥80 years constituted only 15% in randomised trials on mechanical thrombectomy (MT), but is a considerable higher proportion in clinical practice. Association of clinical variables collected before MT with functional outcome has not been independently described in these patients, while being important in the decision of patient eligibility for MT. Methods: We included patients consecutively at a single centre (2017–2021) categorised as octogenarians (age: 80–89 years) or nonagenarians (age: 90–99 years). Functional outcome at 90 days was defined as fair (modified Rankin Scale (mRS) 0–3) or poor (mRS 4–6). Clinical variables collected before MT were analysed for association with shift of mRS in a poor direction. Significant predictors were used to produce a risk score of fair outcome. Significance was set at the p < 0.05 level. Results: Nonagenarians (n = 43, 15.5%) compared to octogenarians (n = 235, 84.5%) less likely achieved fair outcome (20.9% vs. 46.0%, p < 0.01) with higher mortality (65.1% vs. 31.9%, p < 0.01). Significant predictors of outcome were: age, adjusted odds ratio (aOR) = 0.91 (95% confidence interval (CI): 0.86–0.97); pre-stroke mRS, aOR = 0.57 (95% CI: 0.44–0.73); National Institute of Health Stroke Scale at admission, aOR = 0.91 (95% CI: 0.87–0.95); Alberta Stroke Program Early Computed Tomography, aOR = 1.23 (95% CI: 1.05–1.45). After bootstrap validation, the area under the curve of the risk score was 0.74 and the optimal cut-off for fair outcome was a score of >7 points. Conclusions: One in two octogenarians achieved fair outcome, however, only one in five nonagenarians had fair outcome. The clinical risk score could be considered as guidance when deciding patient eligibility for MT.