Seabrook joins from Carat where he was associate director on communications planning, and worked for clients including Guinness, Royal Mail and American Express.
He joins during a period of transition for the agency, which has seen several changes of management instigated by chief executive Jed Glanvill.
Glanvill was promoted to the position last October from managing director and set about constructing a new top team.
Ita Murphy was promoted to managing director of the agency in January 2005, Murphy holds the position and also heads up the £200m UK Unilever account.
She was joined by Martyn Rees, who was also promoted to managing director of MindShare's House of Media offering.
Seabrook said: "I was really impressed with the dynamic management team at MindShare and their future vision for the company and, ultimately, I couldn't resist the challenge of the great role they offered as managing partner on BP, Britvic Pepsi and Lenovo."