Background: A common conception is that exercise training is cardioprotective for patients with breast cancer receiving adjuvant chemotherapy, but evidence to support this assertion is limited. This study aims to evaluate the effect of exercise training during adjuvant chemotherapy for breast cancer on long-term structural and functional cardiac outcomes. Methods: This is a follow-up study of two previously performed randomized studies in breast cancer patients; the PACT (N = 204) and PACES (N = 230) study. Cardiac outcomes, including extracellular volume fraction (ECV), left ventricular ejection fraction (LVEF) on cardiac MRI and global longitudinal strain (GLS) on echocardiography, were evaluated in patients allocated to moderate-to high-intensity exercise and non-exercise controls using linear and logistic regression models, adjusted for relevant confounders. Additionally, we explored the influence of self-reported PA during chemotherapy on cardiac outcomes, regardless of treatment allocation. Results: In total, 185 breast cancer survivors were included (mean age 58.9±7.8 years, mean time since treatment 8.5±1.1 years). Mean ECV was 25.3±2.5 in the control group and 24.6±2.8 in the exercise group. Mean LVEF was borderline normal in both (54.6±4.9 and 53.0±7.8) with an LVEF < 50% of 17.1% and 27.8% in control and exercise group, respectively. Compared to control, no significant effect of exercise during chemotherapy on ECV (β = -0.61, 95%CI: -1.55;0.32) or on abnormal ECV (OR = 0.80, 95%CI: 0.26;2.45) was found. Native T1 was statistically significantly lower in the exercise group compared to control (β = -16.75%CI: -31.5 ;-1.93). The odds of having an abnormal native T1 appeared lower in the exercise group (OR 0.58, 95%CI: 0.28;1.17). We found no benefit of exercise for LVEF or GLS (β = -1.82, 95%CI: -4.06;0.42 and β = 0.21, 95%CI: -0.87;1.28), nor on the likelihood of having an abnormal LVEF or GLS (OR = 1.78, 95%CI: 0.79;4.16); OR = 1.21, 95%CI: 0.56;2.63), respectively. Higher self-reported physical activity levels during chemotherapy tended to be associated with better cardiac outcomes. Conclusions: Exercise training during chemotherapy was not associated with long-term cardioprotection in patients with early-stage breast cancer. The high prevalence of cardiac abnormalities years post-chemotherapy suggests the need to include cardiac assessment in long-term follow-up programs for breast cancer survivors and calls for more research on cardioprotective measures during adjuvant chemotherapy, including alternative exercise dosing regimens and pharmacological adjuncts. Clinical trial information: NTR7247.