Background. Using UK Biobank data, this study sought to explain the causal relationship between alcohol intake and cognitive decline in middle and older aged populations. Methods. Data from 13 342 men and women, aged between 40 and 73 years were used in regression analysis that tested the functional relationship and impact of alcohol on cognitive performance. Performance was measured using mean reaction time (RT) and intra-individual variation (IIV) in RT, collected in response to a perceptual matching task. Covariates included body mass index, physical activity, tobacco use, socioeconomic status, education and baseline cognitive function. Results. A restricted cubic spline regression with three knots showed how the linear (β1 = −0.048, 95% CI: −0.105 to −0.030) and non-linear effects (β2 = 0.035, 95% CI: 0.007–0.059) of alcohol use on mean RT and IIV in RT (β1 = −0.055, 95% CI: −0.125 to −0.034; β2 = 0.034, 95% CI: 0.002–0.064) were significant adjusting for covariates. Cognitive function declined as alcohol use increased beyond 10 g/day. Decline was more apparent as age increased. Conclusions. The relationship between alcohol use and cognitive function is non-linear. Consuming more than one UK standard unit of alcohol per day is detrimental to cognitive performance and is more pronounced in older populations.