Background Recent data has acknowledged atrial induced functional mitral valve regurgitation (MR) in the setting of atrial fibrillation (AF) and/or heart failure with preserved ejection fraction (HFpEF) as a distinct type of secondary MR, holding prognostic significance. However, evidence on its prevalence is still scarce, especially in the phenotype of mid-range ejection fraction heart failure (HFmEF). Purpose The aim of this study is to evaluate the occurrence of left atrial (LA) enlargement and MR in AF patients with or without heart failure with preserved or mid-range ejection fraction. Methods This retrospective study included 750 consecutive patients with AF admitted to a tertiary hospital from January 2018 to June 2019. We excluded patients with primary valvular disease and HF with reduced EF. MR presence and severity were assessed by evaluating the valve morphology, colour flow imaging and, when feasible, vena contracta and PISA methods. We measured LA anteroposterior diameter and used LA dilatation as a surrogate marker for mitral annulus dilatation. Results We evaluated 584 AF patients: mean age 72.22 ± 10.10 years; 58,73% females; 79.75% had HF: 73.13% of them had HFpEF and 26.87% had HFmEF. Compared to those without HF, patients with HF had a relative risk (RR) of associating LA enlargement of 5.37 (95%CI = 3.05-9.48, p < 0.001) and a RR of associating MR of 1.47 (95%CI 1.08-2.00, p = 0.01). Mean LA diameter was higher in the HF group, compared to non-HF (47.06 ± 7.26 mm vs 40.91 ± 7.10 mm, p < 0.001). MR severity was more likely associated with HF (RR = 1.68, 95%CI = 1.46-1.94, p < 0.001). When comparing results between the two HF subgroups, patients with HFmEF had a higher mean LA diameter than those with HFpEF (48.52 ± 5.68 mm vs 46.36 ± 7.57 mm, p = 0.011), without associating a significant difference in the MR prevalence (72.97% vs 73.98%, p = 0.94). The presence of a dilated LA was directly correlated with MR in the HF group (RR = 1.94, 95%CI = 1.18-3.20, p = 0.023), but not in those without HF (RR = 1.04, 95%CI = 0.57-1.90, p = 0.89). In HF patients, permanent AF associated the highest prevalence of LA dilatation (96.67%) and MR (81.73%) in contrast to paroxysmal AF (81.10%, p < 0.01, respectively 63.43%, p = 0.0002). Conclusions LA dilatation, the presence and severity of MR correlated with AF and HF, especially in permanent AF patients. In patients without HF, LA dilatation did not correlate with the presence of MR. MR prevalence was similar in patients with HFmEF and HFpEF, irrespective of a higher degree of LA dilatation in HFmEF. Our results suggest that the pathophysiological mechanisms involved in LA enlargement and MR are different for different phenotypes of AF in patients with or without HF.