Previous systematic review and meta-analysis indicates that rehabilitation within a week of intensive care unit (ICU) admission benefits physical function in critically ill patients. This updated systematic review and meta-analysis aim to clarify effects of initiating rehabilitation within 72 h of ICU admission on long-term physical, cognitive, and mental health. We systematically searched the MEDLINE, Cochrane Central Register of Controlled Trials, and Igaku Chuo Zasshi for randomized controlled trials (RCTs) between April 2019 and November 2022 to add to the previous review. Two investigators independently selected and extracted data. Pooled effect estimates for muscle strength, cognitive function, mental health after discharge, and adverse events were calculated. Evidence certainty was assessed via Grading of Recommendations, Assessment, Development, and Evaluations. Eleven RCTs were included in the meta-analysis. Early rehabilitation may improve muscle strength (three trials; standard mean difference [SMD], 0.16; 95% confidence interval [CI], −0.04–0.36) and cognitive function (two trials; SMD, 0.54; 95% CI, −0.13–1.20). Contrastingly, early mobilization showed limited impact on mental health or adverse events. In summary, initiating rehabilitation for critically ill patients within 72 h may improve physical and cognitive function to prevent post-intensive care syndrome without increasing adverse events. The effect on mental function remains uncertain.