A cognitive-energetic account of individual differences in working memory capacity (WMC) and sustained attention performance is proposed suggesting that variation in the voluntary control of the intensity of attention (intrinsic alertness) is critical for the relation between WMC and attention control. Four experiments examining individual differences were conducted to test this account. The results suggested that WMC was consistently related to the slowest reaction times in conditions where the interstimulus interval (ISI) was varied or was fixed at a long interval. Variation in WMC was not related to performance when the ISI was fixed at a short interval which is thought to decrease demands on intrinsic alertness. The current results are consistent with the hypothesis that normal variation in WMC and sustained attention performance are partially the result of individual differences in intrinsic alertness whereby low WMC individuals are less able to consistently control the intensity of attention than high WMC individuals. Other possible reasons for the relation between WMC and sustained attention performance such as differences in goal activation, speed of goal activation, goal maintenance during a trial, or sustaining goal maintenance across the duration of the task were associated with weaker and inconsistent evidence. Collectively we suggest that the current cognitive-energetic account can be used to understand individual differences in WMC and attention control and their relations with other cognitive abilities.