Aims Cancer therapy‐related cardiac dysfunction (CTRCD) is commonly reported, but its histopathology, mechanisms, and risk factors are not known. We aimed to clarify the histopathology and mechanisms of CTRCD to identify risk factors. Methods and results We performed myocardial histopathological studies on 13 endomyocardial biopsies from CTRCD patients, 35 autopsied cancer cases with or without cardiac dysfunction, and controls without cancer (10 biopsies and 9 autopsies). Cardiotoxicity risk scores were calculated based on medication; and patient‐related risk factors, fibrosis, and cardiomyocyte changes were scored; and p53 and H3K27ac histone modification were evaluated by histological score (H‐score). In the biopsy cases, all histopathological changes and the p53 evaluation were significantly higher in the CTRCD group than in the controls [p53 H‐score; 63 (9.109) vs. 33 (5.099), P < 0.05]. In patients with a short time between drug and disease onset (<4.2 years), fibrosis and p53 positively correlated (r = 0.76, P < 0.05), and in those with late onset disease (>4.2 years), cellular abnormalities and p53 trended to a positive correlation and cardiotoxicity risk scores and p53 positively correlated (r = 0.95, P < 0.05). A year after biopsy, the short‐term group had significant recovery of ejection fraction compared with the long‐term group (P < 0.05). The CTRCD group had a significantly worse overall survival prognosis than the control group [hazard ratio 7.61 (95% confidence interval 1.30–44.6), P < 0.05]. Autopsy cases with cancer treatment also had a high grade of histopathological changes, with even more severe changes in patients with cardiac dysfunction, and had increased p53 and H3K27ac expression levels, compared with controls. H‐scores of p53 and H3K27ac showed a positive correlation in the CTRCD group in biopsy cases (r = 0.62, P < 0.05) and a positive correlation in autopsy cases. Conclusions Our results indicate distinct morphological characteristics in myocardial histopathology associated with CTRCD. p53 and H3K27ac histone modification could be sensitive markers of CTRCD and suggest a mechanistic involvement of epigenetic changes.