The mental number line (MNL) hypothesis is that numbers are mentally represented in spatial format, particularly in left-to-right orientation among Westerners. The MNL has received support from various paradigms, but it remains controversial as it is challenged by alternative models. Here we used an individual differences approach to assess spatial-numerical associations (SNAs) across a variety of tasks. The MNL hypothesis predicts correlations across SNA tasks because they should tap a common MNL representation. Control tasks were included to account for effects not specific to SNAs. Correlation analyses revealed significant associations across several SNA tasks, even when controlling for general cognitive abilities or individual differences in response time (RT). These findings provide unique support for the MNL hypothesis, and begin to shed insight on potential explanations that may contribute to variation in the strength of the correlations among SNA tasks.