Background The temporal progression states of the molecular and structural substrate in atrial fibrillation (AF) are not well understood. We hypothesized that these can be detected by AF electrograms and magnetic resonance imaging parametric mapping. Methods and Results AF was induced in 43 dogs (25–35 kg, ≥1 year) by rapid atrial pacing (RAP) (3–33 weeks, 600 beats/min), and 4 controls were used. We performed high‐resolution epicardial mapping (UnEmap, 6 atrial regions, both atria, 130 electrodes, distance 2.5 mm) and analyzed electrogram cycle length, dominant frequency, organization index, and peak‐to‐peak bipolar voltage. Implantable telemetry recordings were used to quantify parasympathetic nerve activity over RAP time. Magnetic resonance imaging native T1, postcontrast T1, T2 mapping, and extracellular volume fraction were assessed (1.5T, Siemens) at baseline and AF. In explanted atrial tissue, DNA oxidative damage (8‐hydroxy‐2′‐deoxyguanosine staining) and percentage of fibrofatty tissue were quantified. Cycle length and organization index decreased ( R =0.5, P <0.05; and R =0.5, P <0.05; respectively), and dominant frequency increased ( R =0.3, P n.s.) until 80 days of RAP but not thereafter. In contrast, voltage continued to decrease throughout the duration of RAP ( R =0.6, P <0.05). Parasympathetic nerve activity increased following RAP and plateaued at 80 days. Magnetic resonance imaging native T1 and T2 times increased with RAP days ( R =0.5, P <0.05; R =0.6, P <0.05) in the posterior left atrium throughout RAP. Increased RAP days correlated with increasing 8‐hydroxy‐2′‐deoxyguanosine levels and with fibrosis percentage ( R =0.5, P <0.05 for both). Conclusions A combination of AF electrogram characteristics and T1/T2 magnetic resonance imaging can detect early‐stage AF remodeling (autonomic remodeling, oxidative stress) and advanced AF remodeling due to oxidative stress and fibrosis.