Byrne, currently WSW UK public affairs chief executive and Brain, BSMG UK chief executive, will become joint UK chief executives of the new entity, when it merges in October.
Tim Sutton, BSMG Europe chairman, will stay on as interim chairman of the new company but will stand down early next year.
Before then, Sutton will chair a newly created UK intergration committee to drive through the merger.
Sutton says he cannot rule out job cuts when the two companies integrate.
He said "I cannot say hand on heart that they will not be job losses due to the merger."
The new company means the end of the BSMG brand, which will roll into WSW. This includes a merger of the two companies' public affairs functions, BSMG's GJW, which is headed by principals Wilf Weeks, Andrew Gifford and Nigel Clarke, and Byrne's WSW PA shop.
Also to be integrated will be the financial operations, BSMG's Square Mile, headed by Tim Jackaman, and WSW's financial, broadcasting and interactive division headed by Tessa Curtis.
A search is underway to find new premises, by spring next year, for all London-based staff of the merged company.