Most everyday activities involve delayed intentions referring to different event structures and timelines. Yet, past research has mostly considered prospective memory (PM) as a dual-task phenomenon in which the primary task to fulfill PM intentions is realized within an ongoing secondary task. We hypothesized that these simplified simulations of PM may have obscured the role of spatial relational processing that is functional to represent and meet the increased temporal demands in more complex PM scenarios involving multiple timelines. To test this spatiotemporal hypothesis, participants monitored four digital clocks, with PM deadlines referring either to the same clock (single-context condition) or different clocks (multiple-context condition), along with separate tests of spatial ability (mental rotation task) and executive functioning (working memory updating). We found that performance in the mental rotation task incrementally explained PM performance in the multiple-context, but not in the single-context, condition, even after controlling for individual differences in working memory updating and ongoing task performance. These findings suggest that delayed intentions occurring in multiple ongoing task contexts reflect independent contributions of working memory updating and mental rotation and that spatial relational processing may specifically be involved in higher cognitive functions, such as complex PM in multiple contexts or multitasking.