When reasoning about sequences of events, English-speaking adults often invoke a “mental timeline,” stretching from left (past) to right (future). Although the direction of the timeline varies across cultures, linear representations of time are argued to be ubiquitous and primitive. On this hypothesis, we might predict that children should spontaneously invoke a timeline when reasoning about time. However, little is known about how the mental timeline develops. Here, we use a sticker placement task to test whether 3- to 6-year-olds spontaneously produce linear, spatial representations of time. We find that, while English-speaking children under age five rarely adopt such representations without prompting, a spatial prime increases the percentage of 4-year-olds producing linear, ordered, and left-to-right representations of time by a factor of three. Our findings suggest that, while preschoolers do not spontaneously use a mental timeline, they can readily form mappings between time and space to represent temporal sequences.