Lavender will oversee UK operations and report to TBG's chief executive and founder, Simon Mansell.
The agency is currently expanding abroad, having doubled its turnover in the past 12 months, and is now managing campaigns for 90 companies in more than 40 countries.
Mansell said: "Kate has a wealth of experience in driving customer service, sales and operational excellence, and will provide a fresh perspective on how to tap into new markets and manage growth effectively for our high performing business."
Lavender has spent more than 10 years in senior sales, customer service and operations roles within companies including Airmiles, Sony, AXA PPP Healthcare and Lastminute.com.
At Sony, she was responsible for a multichannel retail project, creating new customer propositions and business models.