This study aimed to verify the mediating roles of major satisfaction and career decision-making levels in the impact of major selection motivation on career preparation behavior among university students. For this study, a survey was conducted with students from a university in South Korea, and data from 319 responses were used for analysis. The collected data were analyzed using IBM SPSS 29.0, including frequency analysis, correlation analysis, and bootstrapping through the PROCESS macro model 4 to test the mediating effects. The main results are as follows: First, motivation for choosing a major, major satisfaction, career decision-making levels, and career preparation behaviors were all positively correlated with each other. Second, we examined the mediating effect of major satisfaction on the relationship between motivation for major selection and career preparation behaviors. It was found that major satisfaction fully mediated the relationship between intrinsic motivation for major selection and career preparation behaviors. On the other hand, major satisfaction partially mediates the relationship between extrinsic motivation for major selection and career preparation behaviors. Third, we tested the mediating effect of career decision-making levels on the relationship between extrinsic motivation for major selection and career preparation behavior; however, the mediating effect of career decision-making levels was not significant. In summary, this study confirmed that university students' motivation to choose a major has a significant effect on career preparation behavior through the mediating effect of major satisfaction. Lastly, the significance and limitations of this study and the direction for future research were presented.