Spatial skills have been consistently linked to mathematics achievement in older students and adults, but we know little about their relationship to mathematics achievement in elementary school. This study examined how spatial skills influenced the development of number sense, and subsequent mathematics competency, as students progressed from the 2nd to the 4th grade. Gender, verbal working memory (VWM), and socioeconomic status (SES) have also been found to predict number sense development and to be linked to spatial skills; as such, they were included as covariates in this study. Participants were 304 second graders who were assessed at 5 points between 2nd and 4th grade. Two growth mixture models (spatial skills as time-invariant and time-variant covariates) were tested to determine whether different developmental trajectories were needed to explain the development of number sense. Both models revealed the presence of 2 latent classes. The classes differed in their initial level and in their growth rate, with the higher performing class beginning the second grade at an advantage and increasing that advantage over time. SES, VWM, and spatial skills influenced latent class membership and subsequent mathematics competency. SES, spatial skills, and VWM, but not gender, predicted the intercept but differences were found in predictors of the slope of number sense. The impact of number sense changed over time and differed as a function of latent class having an earlier impact on the higher performing class.