Fair, who is credited with six years online retail experience, is currently commercial director at Yahoo! UK, where he oversees revenue generation across the search giant's cars, finance and business development services, and manages accounts for eBay, Amazon and Moneysupermarket.com.
Harkness, a non-executive director at cross-media publishing firm Magicalia Media and Chrysalis Venture Capital Trust, is also a founding director of Keyways Publishing.
Prior to this, he was chairman of IT analysis company Butler Group, before it was sold to Datamonitor. In addition, he currently holds directorships at Advance Media Information, Optima UK and Precise Media Monitoring.
In July, TMN almost doubled its annual post-tax profits to £2.73m from £1.53m, off the back of revenue growth of 79% to £16.1m. The group, which was formerly known as Themutual.net, lists Sky, T-Mobile and British Gas among its clients.
Mark Smith, chief executive officer of TMN, said: "TMN is at an exciting stage in its development and the combined experience of the new directors will support the group as it continues to grow."