Background and Aim: The present study was conducted to investigate the effectiveness of a preschool education program on the development of mathematical thinking skills and academic achievement in mathematics among first-grade elementary school students. Methods: The study employed a causal-comparative research design. The statistical population consisted of first-grade elementary school students in Eslamabad-e Gharb County during the 2023–2024 academic year. The sample included two groups (30 students each), one of which had attended preschool while the other had not. Due to the limited number of students who had not attended preschool, this group was selected using convenience sampling, whereas the group that had attended preschool was selected randomly and matched with the first group. Data were collected using the KeyMath Mathematical Thinking Skills Questionnaire and a researcher-made mathematics academic achievement test. The research hypotheses were tested using multivariate analysis of variance (MANOVA). Results: The findings indicated significant differences between the mean scores of the two groups on the counting, addition, and subtraction subscales of the mathematical thinking skills test, as well as on mathematics academic achievement. In other words, students who had attended preschool demonstrated higher performance. However, no significant difference was observed between the two groups on the estimation subscale. Conclusion: Therefore, it is recommended that the importance of preschool education for parents of children in preschool ages be emphasized through educational workshops.