Background Functional mitral regurgitation (FMR) in type 2 diabetes mellitus (T2DM) patients induced by left ventricular (LV) enlargement and mitral valve abnormality may aggravated the impairment in left atrial (LA) compliance. Thus, this study aimed to depict how FMR and LV dysfunction affect LA compliance in T2DM patients with FMR. Materials and methods A total of 148 patients with T2DM and 49 age- and sex-matched normal controls underwent cardiac magnetic resonance examination. LA longitudinal strain and LA and LV functional indices were compared among controls and different T2DM patients. The multivariate analysis was used to identify the independent indicators of LA longitudinal strain. Results T2DM Patients without FMR had a lower total LA empty fraction (LAEF) compared with the controls (all P < 0.05). T2DM patients with mild and moderate FMR showed increased LA volume (LAV) and LV volume while decreased LAEF, LA strain, and LV ejection fraction (P < 0.05). T2DM patients with severe FMR showed markedly increased LAV and LV volume while decreased LAEF, LA strain, and LVEF (P < 0.05). In T2DM patients with FMR, reservoir strain (epsilon s) was independently correlated with LV end-diastolic volume (LVEDV) (beta = - 0.334) and regurgitation degree (beta = - 0.256). The passive strain (epsilon e) was independently correlated with regurgitation degree (beta = - 0.297), whereas the active strain (epsilon a) was independently correlated with LVESV (beta = - 0.352) and glycated haemoglobin (beta = - 0.279). Conclusion FMR may aggravate LA and LV dysfunction in T2DM patients. Regurgitation degree was an independent determinant of the epsilon s and the epsilon e, LVEDV was an independent determinant of the epsilon s, and LVESV was an independent determinant of the epsilon a in T2DM patients with FMR.