Barrett's time at Marketing comprised two stints. The latter began in 2005 when she returned as deputy editor before becoming editor in 2007.
During her tenure as editor, Barrett was responsible for several groundbreaking initiatives. These included a redesign of the magazine in 2008, when it expanded from being solely a weekly print title to encompass a comprehensive online presence.
She also won accreditation for Marketing from the Carbon Trust and introduced The Little Black Book guide to the UK's top marketers.
Barrett, who has been on maternity leave since December, joins JWT next month. Her successor will be announced in the next few weeks.