De Florio joins Andrew Deitchman, who moved to Mother from WPP's Red Cell in June, and the Swedish creative partnership of Linus Karlsson and Paul Malmstrom, who arrived from Fallon last month.
He comes with ten years' experience of heading Nike's global advertising, during which time the company produced landmark commercials including the Cannes Grand Prix winner "tag".
He previously worked for Lowe & Partners and TBWA/Chiat/Day New York.
The Mother partner Mark Waites said: "Partnering with clients is turning into something of a tradition, and now Rob has also come through from a client background.
"He is absolutely unbelievable and knows exactly what clients want, which is the kind of person we need."
Mother hopes to launch its New York-based US agency in early November.
Mother was launched in London six years ago by Waites, Robert Saville, Andrew Medd and Stef Calcraft. Like de Florio, Medd came from the client side, having worked at Coca-Cola.