Complex sentence: He acted as both the President and CEO Irwin Stayed with MediaVest Worldwide for 6 years until September 1999 when he joined WPP plc as the Chairman and CEO of MindShare Worldwide.
Split into simpler sentences: He acted as both the President and CEO. Irwin stayed with MediaVest Worldwide for 6 years until September 1999 when he joined WPP plc as the Chairman and CEO of MindShare Worldwide.