The agency has been hired on an ongoing retainer and will be responsible for media and analyst relations, content creation and event management.
Donky enables brands to communicate with consumers through mobile apps and smart SMS.
Lyndsey Treharne, marketing and product director at Donky, said: "We spoke to a number of agencies during the pitch process but Liberty really stood out with its clear understanding of what Donky was all about, our objectives and how we wanted to get there.
"We needed an agency that could run strategic and precise PR campaigns and Liberty really fitted that profile."
Dee Gibbs, founder and global CEO at Liberty, said: "Liberty’s heritage in mobile technology and experience reaching core vertical markets means it is well placed to offer this support and act as a trusted partner."