Objective Primary age‐related tauopathy (PART) refers to tau neurofibrillary tangles restricted largely to the medial temporal lobe in the absence of significant beta‐amyloid plaques. PART has been associated with cognitive impairment, but contributions from concomitant limbic age‐related TDP‐43 encephalopathy neuropathologic change (LATE‐NC) are underappreciated. Methods We compare prevalence of LATE‐NC and vascular copathologies in age‐ and Braak‐matched patients with PART (n = 45, Braak stage I–IV, Thal phase 0–2) or early stage Alzheimer disease neuropathologic change (ADNC; n = 51, Braak I–IV, Thal 3–5), and examine their influence on clinical and cognitive decline. Results Concomitant LATE‐NC and vascular pathology were equally common, and cognition was equally impaired, in PART (Mini‐Mental State Examination MMSE = 24.8 ± 6.9) and ADNC (MMSE = 24.2 ± 6.0). Patients with LATE‐NC were more impaired than those without LATE‐NC on the MMSE (by 5.8 points, 95% confidence interval CI = 3.0–8.6), Mattis Dementia Rating Scale (DRS; 17.5 points, 95% CI = 7.1–27.9), Clinical Dementia Rating, sum of boxes scale (CDR‐sob; 5.2 points, 95% CI = 2.1–8.2), memory composite (0.8 standard deviations SD, 95% CI = 0.1–1.6), and language composite (1.1 SD, 95% CI = 0.2–2.0), and more likely to receive a dementia diagnosis (odds ratio = 4.8, 95% CI = 1.5–18.0). Those with vascular pathology performed worse than those without on the DRS (by 10.2 points, 95% CI = 0.1–20.3) and executive composite (1.3 SD, 95% CI = 0.3–2.3). Cognition declined similarly in PART and ADNC over the 5 years preceding death; however, LATE‐NC was associated with more rapid decline on the MMSE (β = 1.9, 95% CI = 0.9–3.0), DRS (β = 7.8, 95% CI = 3.4–12.7), CDR‐sob (β = 1.9, 95% CI = 0.4–3.7), language composite (β = 0.5 SD, 95% CI = 0.1–0.8), and vascular pathology with more rapid decline on the DRS (β = 5.2, 95% CI = 0.6–10.2). Interpretation LATE‐NC, and to a lesser extent vascular copathology, exacerbate cognitive impairment and decline in PART and early stage ADNC. ANN NEUROL 2022;92:425–438