A great deal of prior research has examined the relation between working memory capacity (WMC) and attention control. The current study explored the role of arousal in individual differences in WMC and attention control. Participants performed multiple WMC and attention control tasks. During the attention control tasks participants were periodically probed regarding their current attentional state and both baseline and task-evoked pupillary responses were recorded as indicators of tonic arousal and phasic arousal because of attentional effort, respectively. Latent variable analyses demonstrated that variability in both baseline pupil diameter and task-evoked responses was related to WMC, attention control, and off-task thinking. Furthermore, structural equation models suggested that variability in both baseline pupil diameter and task-evoked pupillary responses predicted off-task thinking, which in turn predicted variation in WMC and attention control. These results provide important evidence linking moment-to-moment fluctuations in arousal to individual differences in WMC and attention control.