Background: Nonstenotic carotid plaque and undetected atrial fibrillation are potential mechanisms of embolic stroke of undetermined source (ESUS), but it is unclear which is more likely to be the contributing stroke mechanism. We explored the relationship between left atrial enlargement (LAE) and nonstenotic carotid plaque across age ranges in an ESUS population. Methods: A retrospective multicenter cohort of consecutive patients with unilateral, anterior circulation ESUS was queried (2015 to 2021). LAE and plaque thickness were determined by transthoracic echocardiography and computed tomography angiography, respectively. Descriptive statistics were used to compare plaque features in relation to age and left atrial dimensions. Results: Among the 4155 patients screened, 273 (7%) met the inclusion criteria. The median age was 65 years (interquartile range [IQR] 54–74), 133 (48.7%) were female, and the median left atrial diameter was 3.5 cm (IQR 3.1–4.1). Patients with any LAE more frequently had hypertension (85.9% versus 67.2%, P <0.01), diabetes (41.0% versus 25.6%, P =0.01), dyslipidemia (56.4% versus 40.0%, P =0.01), and coronary artery disease (22.8% versus 11.3%, P =0.02). Carotid plaque thickness was greater ipsilateral versus contralateral to the stroke hemisphere in the overall cohort (median 1.9 mm [IQR 0–3] versus 1.5 mm [IQR 0–2.6], P <0.01); however, this was largely driven by the subgroup of patients without any LAE (median 1.8 mm [IQR 0–2.9] versus 1.5 mm [IQR 0–2.5], P <0.01). Compared with patients ≥70 years, younger patients had more carotid plaque ipsilateral versus contralateral (mean difference 0.42 mm±1.24 versus 0.08 mm±1.54, P =0.047) and less moderate-to-severe LAE (6.3% versus 15.3%, P =0.02). Conclusions: Younger patients with ESUS had greater prevalence of ipsilateral nonstenotic plaque, while the elderly had more LAE. The differential effect of age on the probability of specific mechanisms underlying ESUS should be considered in future studies.