Response inhibition, defined as the ability to withhold a response, is considered to be a core deficit in various mental illnesses. Measures of response inhibition have been used to define functional deficits, as markers of genetic risk, in neuroimaging studies, and for diagnostic purposes in these disorders. However, the magnitude of the deficit across psychopathologies has not been systematically assessed. We conducted a systematic review and meta-analysis of performance on commonly used measures of the ability to withhold a response: go/no-go task, Conners' continuous performance task (CCPT), and sustained attention to response task (SART). The primary variable of interest in each of these tasks was commission errors (CE), which provides an index of one's ability to correctly withhold a response. In addition, we examined omission errors (OE) which are an index of sustained attention; and mean reaction time (RT; MRT). Three-hundred and 18 studies in 11 different psychiatric disorders met inclusion criteria. Weighted mean effect sizes (ESs) were calculated to measure the magnitude of the deficit. In general, we found low-to-medium ESs for commission errors ranging from g = -0.10 for anxiety disorder to medium ESs of g = 0.52 for bipolar disorder. Small-to-medium deficits in withholding were found in various disorders. Results indicate that deficits in withholding are insufficiently sensitive or specific to be used individually as a diagnostic measure or biomarker in most disorders.