Long-term cognitive impairment is common among ICU survivors, but its natural history remains unclear. In this systematic review, we report the frequency of cognitive impairment in ICU survivors across various time points following ICU discharge that were extracted from 46 of the 3350 screened records. Prior studies used a range of cognitive instruments including subjective assessments (10 studies), single or screening cognitive test such as Mini-Mental State Examination (MMSE) or Trail Making Tests A and B (23 studies), and comprehensive cognitive batteries (26 studies). The mean [95% CI] prevalence of cognitive impairment was higher with objective rather than subjective assessments (54 [51-57]% versus 35 [29-41]% at 3 months following ICU discharge), and when comprehensive cognitive batteries rather than MMSE were used (ICU discharge: 61 [38-100]% versus 36 [15-63]%; 12 months post ICU discharge: 43 [10-78]% versus 18 [10-20]% versus). Patients with ARDS had higher prevalence of cognitive impairment than mixed ICU patients at ICU discharge (82 [78-86]% versus 48 [44-52]%). While some studies repeated tests at more than one time point, the time intervals between tests were arbitrary and dictated by operational limitations of individual studies or chosen cognitive instruments. In summary, the prevalence and temporal trajectory of ICU-related cognitive impairment varies depending on the type of cognitive instrument used and the etiology of critical illness. Future studies should utilize modern comprehensive batteries to better delineate the natural history of cognitive recovery across ICU patient sub-groups and determine which acute illness and treatment factors are associated with better recovery trajectories.