Background Electroconvulsive therapy (ECT) is provided for patients with severe and often life-threatening illness, who lack decision making capacity to consent to treatment (DMC-T) in clinical settings. Objective The aim of this study is to summarize previous studies investigating clinical outcomes of ECT in patients lacking DMC-T. Methods A systematic review and meta-analysis of studies reporting clinical outcomes of ECT in patients lacking DMC-T with any psychiatric diagnoses was conducted. The primary outcome was clinical improvement. Secondary outcomes were cognitive outcomes and six month readmission rate. Hedges’ g and odds ratios were calculated using a random-effects model. The protocol was registered in Open Science Framework (https://osf.io/rxjkm). Results Of 3552 identified articles, 41 studies (n = 1299) were included. Approximately 80% of patients lacking DMC-T responded to ECT, and part of the patients regained capacity to consent and consented to further treatment with ECT. A total of seven studies (n = 1081) were included for meta-analysis. Patients without DMC-T showed superior clinical improvement and less cognitive side effects compared with those with DMC-T, whereas the groups did not show any difference in readmission rate. Several clinical characteristics at baseline and ECT techniques were significantly different between the groups. Conclusion ECT is equally, if not superiorly, effective in patients lacking DMC-T compared to patients with DMC-T. ECT can potentially enhance patients’ autonomy, without increasing the risk of cognitive side effects. These results support the clinical and ethical legitimacy of ECT provision for patients with the most severe illness who lack DMC-T at start of treatment.