Understanding how early reading and mathematics co-develop is important from both theoretical and pedagogical standpoints. Previous research has provided mixed results. This paper investigates the development of reading and mathematics in a longitudinal sample of N = 355,883 students from the United Kingdom (2005-2019) aged 5 to 12 (49% girls). Results indicate a positive relation between the development of the two domains. In addition, a substantial statistically significant positive association between prior reading scores and subsequent changes in achievement in mathematics was found, whereas changes in reading were smaller for students with a higher prior performance in mathematics. The findings suggest that acquiring good reading skills is highly relevant for developing mathematics skills. Implications for theory and practice are discussed.