Memory for numbers improves with age and experience. We tested the hypothesis that one source of this improvement is a logarithmic-to-linear shift in children’s representations of numeric magnitude. In Experiment 1, we found that linearity of representations improved with age and that the more linear children’s magnitude representations were, the more closely their memory of the numbers approximated the numbers presented. In Experiment 2, we trained children on a linear spatial-numeric association, and we found that children who learned to represent numbers as increasing linearly with numeric magnitude also improved their memory for numbers. These results suggest that linear spatial-numeric associations are both correlated with and causally related to development of numeric memory.