Among the wide range of factors related to the acquisition of statistical knowledge, competence in basic mathematics, including basic probability, has received much attention. In this study, a mediation model was estimated to derive the total, direct, and indirect effects of mathematical competence on statistics achievement taking into account probabilistic reasoning ability. The participants were psychology students enrolled in an undergraduate introductory statistics course. At the beginning of the course, all students completed a questionnaire to measure their proficiency in the basic probabilistic reasoning and mathematics skills required in introductory statistics courses. At the end of the course, the students’ final grades were collected. The hypothesized mediation model was tested using a bootstrapping procedure (with 5000 bootstrap samples). Results showed a significant positive, indirect effect of mathematical competence on students’ final grades, with probabilistic reasoning ability acting as a mediator. This study suggests that interventions designed to promote the mathematical prerequisites necessary to probabilistic reasoning can have a positive effect on achievement in statistics.