Guaraná (Paullinia cupana) extracts are most commonly used in Western markets as putatively psychoactive food and drink additives. This double-blind, randomised, placebo-controlled, parallel groups study assessed the acute effects of either a vitamin/mineral/guaraná supplement or placebo drink in 129 healthy young adults (18–24 years). Participants completed a 10 min version of the Cognitive Demand Battery (comprising: Serial 3s and Serial 7s subtraction tasks, a Rapid Visual Information Processing (RVIP) task, ‘mental fatigue’ scale). Thirty minutes following their drink participants made six consecutive completions of the battery (i.e. 60 min). The vitamin/mineral/guaraná combination resulted in improved task performance, in comparison to placebo, in terms of both increased speed and accuracy of performing the RVIP task throughout the post-dose assessment. The increase in mental fatigue associated with extended task performance was also attenuated by the supplement. This research supports previous findings demonstrating guaraná's cognition enhancing properties and provides evidence that its addition to a multi-vitamin–mineral supplement can improve cognitive performance and reduce the mental fatigue associated with sustained mental effort.