Copathologies play an important role in the expression of the AD clinical phenotype and may influence treatment efficacy. Early-onset AD (EOAD), defined as manifesting before age 65, is viewed as a relatively pure form of AD with a more homogeneous neuropathological substrate. We sought to compare the frequency of common neuropathological diagnoses in a consecutive autopsy series of 96 patients with EOAD (median age of onset = 55 years, 44 females) and 48 with late-onset AD (LOAD) (median age of onset = 73 years, 14 females). The UCSF Neurodegenerative Disease Brain Bank database was reviewed to identify patients with a primary pathological diagnosis of AD. Prevalence and stage of Lewy body disease (LBD), limbic age-related TDP-43 encephalopathy (LATE), argyrophilic grain disease (AGD), hippocampal sclerosis (HS), cerebral amyloid angiopathy (CAA), and vascular brain injury (VBI) were compared between the two cohorts. We found at least one non-AD pathological diagnosis in 98% of patients with EOAD (versus 100% of LOAD), and the number of comorbid diagnoses per patient was lower in EOAD than in LOAD (median=2 versus 3, Mann-Whitney Z = 3.00, p = 0.002). LBD and CAA were common in both EOAD and LOAD (CAA: 86% versus 79%, Fisher exact p = 0.33; LBD: 49% versus 42%, p = 0.48, respectively), although amygdala-predominant LBD was more commonly found in EOAD than LOAD (22% versus 6%, p = 0.02). In contrast, LATE (35% versus 8%, p < 0.001), HS (15% versus 3%, p = 0.02), AGD (58% versus 41%, p = 0.052), and VBI (65% versus 39%, p = 0.004) were more common in LOAD than EOAD, respectively. The number of copathologies predicted worse cognitive performance at the time of death on MMSE (1.4 points/pathology (95%CI -2.5, -0.2) and Clinical Dementia Rating - Sum of Boxes (1.15 point/pathology, 95%CI 0.45, 1.84), across the EOAD and the LOAD cohorts. The effect of sex on the number of copathologies was not significant (p = 0.17). Prevalence of at least one APOE ε4 allele was similar across the two cohorts (52% and 54%) and was associated with a greater number of copathologies (+0.40, 95%CI 0.01, 0.79, p = 0.047), independent of age of symptom onset, sex, and disease duration. Females showed higher density of neurofibrillary tangles compared to men, controlling for age of onset, APOE ε4, and disease duration. Our findings suggest that non-AD pathological diagnoses play an important role in the clinical phenotype of EOAD with potentially significant implications for clinical practice and clinical trials design.