The campaign is part of the agency's ongoing for the charity.
Mindshare won the account without a pitch. It is responsible for the charity's digital media strategy, which includes contextual display, Facebook pay-per-click and re-targeted display, and GroupM marketplace activity.
The campaign will launch next week with the aim of improving awareness among existing audiences, and targeting new ones.
Integrated agency Domain Group currently handles the creative work for the charity.
Emma Douglas, Alzheimer's Research UK, marketing and PR officer, said: "Mindshare have a wealth of digital experience, combined with investment in proprietary tools and targeting techniques, which will really help to build awareness and support for our organisation."
Emma Stears, Mindshare Direct, managing partner, said: "It's fantastic to be involved with such an innovative charity that is at the forefront of advances in research around this disease.
"Using our digital expertise, we hope to widen the understanding of dementia and Alzheimer's Research UK's work in this field, and generate more support for their pioneering research."