Although mental folding is prominently used in the adult spatial literature (Harris, Newcombe, & Hirsh-Pasek, 2013), where it is correlated with math ability and STEM career entry (e.g. Uttal et al., 2013), little is known about its developmental trajectory. The present study seeks to analyze children’s performance on a novel paper folding task to see (1) how children’s mental folding skills develop between 6 and 8 years of age and what children’s errors on the paper folding task tell us about this development, and (2) how children’s performance on our paper folding task is related to their mental rotation skill and a foundational math skill – their number line estimation. Findings showed a significant difference between 6- and 7-year-olds’ performance on the paper folding task, and a significant difference in the number of “hole errors” they made. We found that paper folding performance was correlated with both mental rotation and number line performance when controlling for vocabulary knowledge and age. Importantly, paper folding accuracy predicted number line performance significantly, controlling for mental rotation skill, a vocabulary measure, and age, which has implications on interventions that benefit spatial and general math ability.