The present study examines how attending the German model project "Kindergarten of the Future in Bavaria" (KiDZ), which provided 138 children (aged 3 to 6) with traditional preschool stimulation combined with cognitive and domain-specific stimulation, is associated with children's competencies in mathematics over time to age 12 compared to a control group of 53 children. Controlling for child and family background measures, attending KiDZ was positively associated with mathematical competencies at the age of 12 compared to the control group not attending. Those effects are mediated through grade and preschool quality. Furthermore, results showed that preschool quality was associated with higher competencies in mathematics at age 12, even when controlling for numerical skills at preschool and primary school age. The results suggest that preschool quality influences mathematical development over and above those effects detected previously in preschool. Thus, the academically oriented high-quality preschool intervention, KiDz, lays not only the foundation for a good start in school, but also for later development in secondary school.