Lifetime social engagement could build cognitive reserve and lower the risk of dementia through compensatory effects on brain health. We aimed to investigate whether social interaction at work is protective of later-life cognitive impairment. Data from 9,248 participants of the population-based Norwegian HUNT4 70+ Study (2017-2019), with cognitive assessments at or after age 70, were linked retrospectively to longitudinal registry-based employment information spanning ages 30-65 years. An occupational social interaction score was computed using occupational characteristics from the O*NET database. Multinomial logistic regression was used to estimate the associations between occupational social interaction and dementia and mild cognitive impairment (MCI), while linear regression was used to model the association with cognition using the Montreal Cognitive Assessment (MoCA). The models were subsequently adjusted for confounding variables age, sex, education, and apolipoprotein E-ε4 genotype, as well as for midlife family, health, and lifestyle-related variables collected from national registries and earlier Trøndelag Health Study waves. Higher occupational social interaction was associated with reduced risks of dementia and MCI, and better MoCA performance. Adjusted for confounding from age, sex, education, and apolipoprotein E-ε4, each standard deviation higher occupational social interaction score was associated with a relative risk ratio of 0.89 for dementia (p = .003), 0.88 for MCI (p < .001), and a 0.31-point higher MoCA score (p < .001). Our findings highlight the importance of occupational social interaction in preserving and promoting cognitive health in later life. (PsycInfo Database Record (c) 2025 APA, all rights reserved).