Prior research has shown the importance of the ACT score and high school GPA (HSGPA) in predicting college success. Early college success, as indicated by students' first-year college GPA (FYGPA), plays a pivotal role in later college success and timely degree completion (Demeter et al., 2022; Gershenfeld et al., 2016). The accurate prediction of FYGPA is important in helping colleges identify students who might need academic interventions and supports. The present study examines the predictive validity of ACT scores and HSGPA on cumulative graduating college GPA (CGPA), with FYGPA acting as a mediator. The study concludes that both high school GPA (HSGPA) and ACT scores are significant predictors of college success, particularly first-year college GPA (FYGPA). However, FYGPA mediates the relationship between ACT scores and cumulative GPA (CGPA), but not between HSGPA and CGPA. Importantly, the interaction between HSGPA and ACT scores is a significant predictor of CGPA, with consistent achievement across both measures being crucial for better outcomes. The study emphasizes the value of using both HSGPA and ACT scores for more accurate predictions and tailored academic support. For students, the study suggests the need to focus on both maintaining high grades and performing well on standardized tests to ensure a strong start in college. Teachers can use these insights to better identify students who might need extra help in aligning their academic performance across different achievement measures. For postsecondary professionals, the findings highlight the importance of a comprehensive admissions process that considers both HSGPA and ACT scores.