Response variability has been identified as a useful predictor of executive function and performance in non-clinical samples in the Go/NoGo task. The present study explores the utility of reaction time variability (RTV) and EEG measures as predictors of Go/NoGo performance outcomes and ERP component amplitudes. Forty-four young adults had EEG recorded across eyes-closed (EC) and eyes-open (EO) resting states, and during an auditory Go/NoGo task. The 18 individuals with the lowest/highest RTV were assessed for differences in behavioural outcomes. As expected, individuals with high RTV committed more Go/NoGo errors and had smaller Go P3b and NoGo P3a amplitudes, and greater Go Slow Wave positivity, reflecting inefficient decision-making and response control efforts underlying performance. When RTV and EEG measures were modelled as predictors of Go/NoGo responses, RTV and task-related changes in delta were identified as positive predictors of Go SW amplitude; while RTV and prestimulus delta amplitudes negatively predicted NoGo accuracy rates. Prestimulus delta was also found to solely predict Go mean RT and NoGo SW negativity; effects that were independent of RTV. As delta has been implicated in attention-related mechanisms, these findings suggest that inadequate attention and task engagement underpin the variability in Go/NoGo performance.