His departure was announced by the TBWA\Worldwide president and chief executive, Jean-Marie Dru, and follows the network's decision to shift its global headquarters from London to the US.
Dru believes the effort to drive international new business will be more effective from New York. Rod Wright, the director of development at TBWA\Worldwide, will take over until a permanent, New York-based global chief executive is appointed.
Dru said: "The development of Tequila continues to be vital to our success as a network. We wish Tom much success on his future endeavours and thank him for his contributions."
Following a career in FMCG sales and marketing and retail management, Wass moved into an agency career in 1987, when he joined Orbit International, where he remained until 1992.
He joined the UK arm of Tequila, then owned by BDDP. It went through two takeovers, first by GGT in 1997 and then TBWA in 1998.
Since then, Wass has been concentrating on building the global network, which now comprises 44 offices in 31 countries.