Simpson joined TBA as client services director in 2005, before taking on the role of operations director in 2009, and then director in 2011.
TBA managing director Guy Horner said: "Chad has left the agency for personal reasons after a happy nine years and is taking some time out to consider his next step. He leaves on very good terms and we wish him all the best."
He added that the agency was not looking to appoint another director to replace Simpson, but hopes to make further recruitment announcements in the coming weeks.
Earlier this month, the agency announced four new full-time staff members after a 'strong start' to 2015. The new recruits include James Howitt and Karen Swift who joined TBA as account directors. Howitt moved from WRG, while Swift joined from RPM.
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