Objective: To evaluate the extent to whether declining performance across the Digit Span subtests of the WAIS-IV predicted higher variability on tests requiring vigilant performance. Method: The sample contained 120 participants (53.3% female; mean age 33.5; 81.7% right handed; mean years education 13.6; 57.5% Caucasian, 25.8% Latino-Hispanic, 10% African-American) derived from a mixed clinical sample with a variety of psychiatric and neurologic disorders. Analyses were conducted on archival data from a university mental health clinic. A Digit Span Variability (DSV) score was calculated by subtracting the Digit Span Sequencing from the Digit Span Forward raw score, with lower scores indicating greater decline in performance. Those with DSV scores of zero were excluded from analyses. T-scores from the MMPI-2 and the Conners' CPT-II (CCPT-II) were utilized to assess vigilance on these tasks. Results: DSV significantly correlated with VRIN, and Variability scores on the CCPT-II, but not with other scales from these measures. These scores were entered as predictors into a multiple regression, resulting in a significant model (F(2,117) = 12.54, p < .001) explaining 16.2% of the variance. CCPT-II Variability significantly predicted DSV (β = −.070; p < .001), as did VRIN (β = −.060; p = .002). Conclusion: Participants displaying greater performance decline across Digit Span subtests, showed increasingly variable performance on the MMPI-2 and CCPT-II. Those with lower DSV scores more frequently responded inconsistently to personality questions with similar content, and displayed increasingly inconsistent reaction times when responding to target stimuli over time. Results suggests that the DSV may provide an additional indicator of impaired vigilance, specifically when testing time is limited.