The current study investigated the development of children’s performance on tasks that have been suggested to underlie early mathematics skills, including measures of cardinality, ordinality and intelligence. Eighty-seven children were tested in their first (T1) and second (T2) school year (at age 5 and 6). Children’s performance on all tasks demonstrated good reliability and significantly improved with age. Correlational analyses revealed that performance on some mathematics-related tasks were non-significantly correlated between T1 and T2 (Number line and Number comparison), showing that these skills are relatively unstable. Detailed analyses also indicated that the way children solve these tasks show qualitative changes over time. By contrast, children’s performance on measures of intelligence and non-numerical ordering abilities were strongly correlated between T1 and T2. Additionally, ordering skills also showed moderate to strong correlations with counting procedures both cross-sectionally and longitudinally. These results suggest that, initially, mathematics skills strongly rely on non-mathematical abilities.