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Background: Although the long-term health effects of COVID-19 are increasingly recognised, the societal restrictions during the COVID-19 pandemic hold the potential for considerable detriment to cognitive and mental health, particularly because major dementia risk factors-such as those related to exercise and dietary habits-were affected during this period. We used longitudinal data from the PROTECT study to evaluate the effect of the pandemic on cognition in older adults in the UK., Methods: For this longitudinal analysis, we used computerised neuropsychology data from individuals aged 50 years and older participating in the PROTECT study in the UK. Data were collected from the same participants before the COVID-19 pandemic (March 1, 2019-Feb 29, 2020) and during its first (March 1, 2020-Feb 28, 2021) and second (March 1, 2021-Feb 28, 2022) years. We compared cognition across the three time periods using a linear mixed-effects model. Subgroup analyses were conducted in people with mild cognitive impairment and in people who reported a history of COVID-19, and an exploratory regression analysis identified factors associated with changes in cognitive trajectory., Findings: Pre-pandemic data were included for 3142 participants, of whom 1696 (54·0%) were women and 1446 (46·0%) were men, with a mean age of 67·5 years (SD 9·6, range 50-96). Significant worsening of executive function and working memory was observed in the first year of the pandemic across the whole cohort (effect size 0·15 [95% CI 0·12-0·17] for executive function and 0·51 [0·49-0·53] for working memory), in people with mild cognitive impairment (0·13 [0·07-0·20] and 0·40 [0·36-0·47]), and in people with a history of COVID-19 (0·24 [0·16-0·31] and 0·46 [0·39-0·53]). Worsening of working memory was sustained across the whole cohort in the second year of the pandemic (0·47; 0·44-0·49). Regression analysis indicated that cognitive decline was significantly associated with reduced exercise (p=0·0049; executive function) and increased alcohol use (p=0·049; working memory) across the whole cohort, as well as depression (p=0·011; working memory) in those with a history of COVID-19 and loneliness (p=0·0038; working memory) in those with mild cognitive impairment. In the second year of the pandemic, reduced exercise continued to affect executive function across the whole cohort, and associations were sustained between worsening working memory and increased alcohol use (p=0·0040), loneliness (p=0·042), and depression (p=0·014) in those with mild cognitive impairment, and reduced exercise (p=0·0029), loneliness (p=0·031) and depression (p=0·036) in those with a history of COVID-19., Interpretation: The COVID-19 pandemic resulted in a significant worsening of cognition in older adults, associated with changes in known dementia risk factors. The sustained decline in cognition highlights the need for public health interventions to mitigate the risk of dementia-particularly in people with mild cognitive impairment, in whom conversion to dementia within 5 years is a substantial risk. Long-term intervention for people with a history of COVID-19 should be considered to support cognitive health., Funding: National Institute for Health and Care Research., Competing Interests: Declaration of interests AC declares funding for this Article from the National Institute for Health and Care Research and grants from Synexus, reMYND, and Novo Nordisk. AH declares payment made to consultancy company Future Cognition for support and development of computerised cognitive assessment software. JC declares funding from the National Institute of General Medical Sciences and the National Institute on Aging; consulting fees from AB Science, Acadia Pharmaceuticals, Alkahest, Alpha Cognition, ALZpath, Annovis Bio, AriBio, Artery Therapeutics, Avanir Pharmaceuticals, Biogen, Biosplice Therapeutics, Cassava Sciences, Cerevel Therapeutics, Clinilabs, Cortexyme, Diadem Biotherapeutics, EIP Pharma, Eisai, Gatehouse Bio, GemVax & KAEL, Genentech, Green Valley, Grifols, Janssen Pharmaceuticals, Karuna Therapeutics, Lexeo Therapeutics, Lilly, Lundbeck, LSP Dementia, Merck, NervGen Pharma, Novo Nordisk, Oligomerix, Otsuka Pharmaceutical, Pharmacotrophix, PRODEO Institute, Prothena Biosciences, reMYND, Renew Pharmaceuticals, Resverlogix, Roche, Signant Health, Suven Life Sciences, Unlearn. AI, Vaxxinity, Vigil Neuro, and Zai Lab; participation on advisory boards for Acadia Pharmaceuticals, Biogen, Genentech, Grifols, Janssen Pharmaceuticals, Karuna Therapeutics, Otsuka Pharmaceutical, reMYND, Roche, and Signant Health; and stock in Adamas Pharmaceuticals, Acumen Pharmaceuticals, Alkahest, Alzheon, Annovis Bio, Behren Therapeutics, Bioasis Technologies, MedAvante, and United Neuroscience. CB declares grants from Synexus, reMYND, and Novo Nordisk; consultancy fees from Tau Therapeutics, Acadia Pharmaceuticals, Johnson & Johnson, Suven Life Sciences, Sunovion, Exciva, Roche, AbbVie, Orion Pharma, BioExcel, AARP, and Lilly; and honoraria from Bristol Myers Squibb, Axome Therapeutics, Tau Therapeutics, and Biogen. All other authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 license. Published by Elsevier Ltd.. All rights reserved.)