She replaces Suzie Shaw, who was promoted to client services director last month.
McKeen, 31, will report to TBWA\London's chief executive, Matt Shepherd-Smith, and be responsible for running the agency's new-business function and marketing efforts. She will also be in charge of the marketing of TBWA\London's brand entertainment arm, Stream\, as well as working closely with TBWA's partner agencies, Tequila\ and Agency.com.
McKeen joined Lowe London in 2003, and has headed the Sure, Peperami and Hellmann's accounts. A 2006 Campaign Face to Watch, she started her career as a graduate at Grey in 1997, before moving to BMP DDB in 1999.
Shepherd-Smith said: "Julie is one of the most sought-after people in our business. She is an exceptional talent and will be a natural in this new role. We're delighted that she is joining us."