Although reporting to Chris Komisarjevsky, the agency's New York-based Worldwide chief executive, she is to relocate to London.
B-M spokesman Andrew Wigley said: 'Although she has a worldwide remit she will concentrate on Europe. This reflects the importance of technology in this area and some recent big client wins.'
In March this year, B-M was hired to handle PR for German tech giant SAP, replacing agency Waggener Edstrom.
Other B-M tech clients include Qualcomm, Alcatel and Iridium.
Sinclair joined B-M in 1994 as senior vice-president of its Los Angles office. She has also served as the agency's managing director for international creative management.
In the US she will be replaced by Albert Durig, chairman of the B-M's Latin American tech practice.