Objective: Difficulties with prospective memory are frequently reported following pediatric traumatic brain injury (TBI), but rarely researched. We aimed to (i) investigate time-based prospective memory post-pediatric TBI; (ii) examine whether time-based prospective memory is differentially impacted by the demand placed on working memory; and (iii) explore which components of working memory (viz., central executive, phonological loop, and visuospatial sketchpad) are involved in time-based prospective memory under low and high cognitive load following pediatric TBI. Method: Thirty-nine children and adolescents (20 survivors of moderate–severe TBI and 19 healthy controls) completed (i) a newly developed time-based prospective memory task (TBPMT), embedded in an ongoing lexical-decision task, with two conditions: low and high working memory load; and (ii) tests of working memory components from the Automated Working Memory Assessment. Results: Compared to controls, participants with TBI had significantly lower prospective memory accuracy on the TBPMT across working memory loads (low, high). In contrast, the groups did not differ in time monitoring or accuracy on the ongoing task. In the TBI group, higher scores on tests of the central executive (but not phonological loop or visuospatial sketchpad) were associated with higher prospective memory accuracy and time monitoring (controlling for age). Conclusions: Time-based prospective memory is impaired following moderate–severe pediatric TBI, irrespective of the level of working memory demand. Our findings suggest that children and adolescents with TBI may be at risk of failing to perform future intentions at the right time in daily life, especially for individuals identified as having deficits in central executive functioning.