Time-based prospective memory (PM) refers to performing intended actions at a future time. Participants with time-based PM tasks can be slower to perform ongoing tasks (costs) than participants without PM tasks because internal control is required to maintain the PM intention or to make prospective-timing estimates. However, external control can be gained, and internal control minimized, by checking clocks or by using PM reminders. We present 3 experiments that examined how individuals externalize and internalize control of time-based PM tasks. The control condition performed a lexical decision task only, whereas the PM conditions were additionally required to make a time-based PM response after 11 min. We manipulated whether participants received a reminder, and whether clock checking was discouraged. In Experiments 1 and 3, no cost was found under standard clock check conditions. In contrast, when participants were discouraged from clock checking (Experiments 2 and 3), significant costs were found, accompanied by a decrease in clock checking. PM reminders prompted participants to check the clock, and improved PM accuracy if those reminders were expected. However, there was no evidence that participants could localize the internal or external control of the PM task to after the presentation of an expected reminder (Experiment 3). We conclude that much of the need for internal control can be transferred to the external world by performing a well-practiced task such as clock checking, which reminds participants of the PM task and reduces the internal control required to maintain the intention to perform the PM task.