Separately, Red Urban is understood to have won a global pitch for the Diageo-owned rum brand Captain Morgan.
Jefferies, who will be based in London, will be responsible for overseeing the growth of the network and pushing it as a separate global digital offering to Tribal DDB.
He is joined by Michael Udell, a strategy director at Tribal, who becomes head of planning.
Jefferies will report to Jonathan Nelson, a US-based advisor to Omnicom, and the business will operate with a separate P&L to Tribal DDB. Further hirings are expected over the coming months.
Omnicom bought the Amsterdam-owned agency Red Urban last year for an undisclosed sum. The agency was initially used as a conflict shop for Diageo, after Tribal DDB, which handles Anheuser- Busch in the US, won a global digital pitch for Guinness last year, beating AKQA, Carlson and Agency Republic.
Before joining Tribal DDB, Jefferies was the business development director at the Omnicom-owned Zulu, and has previously held positions at TBWA and Wolff Olins.
Jefferies said: "With clients demanding digitally led communications, we are well poised to take full advantage of the future media environment. The opportunity now exists for agencies that can operate across multiple disciplines, rather than being owned by one."