Novel and unpredictable learning conditions encountered during school-to-college transitions undermine students’ agentic control, emotional resilience, and academic outcomes (Perry, 2003; Schneider & Preckel, 2017). Though motivation treatments can offset these perils (Perry et al., 2005, 2014), boosting motivation-treatment efficacy remains underexamined. In a two-semester online course, a cognitive-reframing booster (vs. no-booster) was administered in Semester 1 following an empirically-valid motivation treatment to assess whether the booster aided students’ Semester 2 academic outcomes (stress, negative emotions, final course grades, course withdrawals). The Semester 1 booster decreased students’ negative emotions, stress, and course withdrawals, and increased final course grades. Booster effects on final course grade interacted with students’ perceived academic control and their first course test, whereby the Booster assisted students most at risk academically (i.e., low PAC; poor test performance). Path analyses indicated the booster à grades path was mediated by reductions in students’ negative emotions, in keeping with Weiner’s (1985, 2014, 2018) attribution theory. These results advance the motivation treatment literature by showing boosters increase the motivation treatment efficacy via theory-based affective processes.