Kelly, who joins Republic in February as its media director, will have a seat on the board and will report to Martin Brooks, the group chief executive of Zulu.
Kelly has been at Unique for three years. Before that, he worked at Tribal DDB.
Bradley Moore, Republic's current media director, will remain at the agency and report to Kelly. Moore will now concentrate on media buying and negotiations.
According to Brooks, Kelly has the strategic and management expertise to develop the 14-strong media department. He said: "Lots of clients have relationships with traditional media agencies that are great at buying but not as good at online strategy. That's where we see a gap in the market."
Separately, Republic has hired Nigel Palmer as the business director on its newly won Mercedes-Benz account. Palmer joins from Tribal DDB, where he spent two years. Before that, he worked at Delaney Lund Knox Warren & Partners.