The agency, which was appointed without a pitch, will initially create a campaign that will launch this summer across TV, print and digital.
Kitcatt Nohr Alexander Shaw, the incumbent, will continue to work on the below-the-line business.
The appointment follows the relaunch of the website two weeks ago. The site, which is looking to move away from reuniting old friends and into standard social networking, has dropped its pay wall and adopted a free subscription model closer to rivals such as Facebook and MySpace.
The new brand identity included a refreshed logo with a close-up of a selection of faces replaced by coloured silhouettes of older people.
HMDG's remit is expected to cover sister sites Friends Reunited Dating and Genes Reunited, a genealogy service. Both sites will remain subscription-based.
Friends Reunited was an early leader in social networking when it launched in July 2000, but has struggled to grow its audience in the wake of free competition.
The website, which was launched by husband and wife Steve and Julie Pankhurst, was acquired by ITV in 2005 for £120 million.
A spokesman for Friends Reunited said: "We don't talk about our media plans but, to avoid confusion, Kitcatt Nohr is our retained agency and we are working on a couple of projects with HMDG."