The present study implemented an individual differences approach in conjunction with response time (RT) variability and distribution modeling techniques to better characterize the cognitive control dynamics underlying ongoing task cost (i.e., slowing) and cue detection in event-based prospective memory (PM). Three experiments assessed the relation between proactive control ability, ex-Gaussian parameter estimates (μ and τ), intraindividual variability in responding (coefficient of variation, CoV), and PM cue detection. Experiment 1 examined these relations using a standard nonfocal PM paradigm. Experiments 2 and 3 further assessed how PM importance and PM cue focality, respectively, influenced performance. Across all experiments, nonfocal PM was associated with increases in all cost measures, but only μ reliably predicted cue detection. Importance instructions and focal PM cues selectively increased and decreased μ cost, respectively, relative to the standard nonfocal condition. These findings suggest that μ cost may reflect a target-checking process that benefits cue detection and produces slowing throughout the entire ongoing task. Additionally, across all experiments proactive control was positively associated with μ cost and cue detection, and generally negatively associated with variability cost (τ and CoV). These findings suggest that natural variation in proactive control ability may affect reliance on more efficacious monitoring processes that facilitates cue detection. Furthermore, variability in responding may have little influence on successful PM. The results from the current study highlight the utility of RT variability and distribution analyses in understanding PM costs and have important implications for extant theories of PM concerning the cognitive control processes underlying cue detection.