Thomas is currently CEO of Asia, the Middle East and Africa and will take up his new role in May.
He will oversee 21 offices across the US, Canada and Latin America, and will remain chairman of I&S BBDO Japan and chairman of Proximity Worldwide.
Jean-Paul Burge will replace him as Asia chairman and chief executive. Burge will step up from his current position as president of Southeast Asia and chief executive of BBDO Singapore.
In his new role Thomas fills a vacancy left by Troy Ruhanen, who became the executive vice president of Omnicom Group in 2013 and is now president and chief executive of TBWA Worldwide.
Thomas did not comment on the strategy he would take or the challenges in the Americas given that the leadership position has been open since September 2013, preferring to speak from a personal perspective.
Thomas told Campaign Asia-Pacific: "I've lived in Asia a long time, in Europe for a long time, but I've never lived in the US, and that will probably be my first challenge.
"I do, however, know our US colleagues really well and most of our MNC clients, while based in North America, are regular travellers to Asia.
"Obviously though, my first order of business is to get to know the region really well, engage with our teams and clients, listen, learn and figure out how I can help."
BBDO has also announced changes to its structure in Greater China. Carol Potter, who has served as president and chief executive there for nearly a decade, will leave the company for personal reasons and return to the UK.
Her next move has not been disclosed, but according to BBDO, it will enable her to spend more time with her family.
In Potter’s place, Tze Kiat Tan has been promoted from managing director to chief executive, Wai Foong Leong has been named chairman and chief creative officer, and Hans Lopez-Vito will serve as chief strategy officer.
Dani Richa, who is currently the chairman and chief executive of BBDO Middle East and North Africa, will assume responsibility for Africa as a whole.
Andrew Robertson, the president and chief executive of BBDO Worldwide, said the changes "demonstrate the benefits of our commitment to strong succession planning and will further strengthen the leadership team".
During his tenure, Thomas led a change of fortune for BBDO in Asia, which Robertson acknowledged as "a sustained transformation of our network".
China has been a particular highlight in that regard, having faced major challenges when Thomas came on board. A number of last year's most significant new business wins came from that market, including GE (also including Singapore), MSD, Master Kang and Inoherb. The network as a whole is one of the region's strongest in terms of creativity and effectiveness.
Last year, BBDO was the Asia-Pacific Network of the Year at Cannes, also winning Network of the Year honours at Spikes Asia, and was the most awarded agency at the AMES awards.
Thomas was also named Agency Head of the Year in Campaign Asia-Pacific's 2014 Agency of the Year awards. Robertson described Thomas as "a truly global citizen who will have a valuable impact on his new time zone".
Reflecting on his nine years leading Asia, Thomas said he was proudest of the strong team he's built.
He said: "We have a wonderful team who have come together as a network. A true network. One that works together on behalf of our clients, and that likes each other. I can honestly say I don't have one single regret professionally."
The industry in Asia has come a long way, too, he added: "When I came to the region, what I said was I wanted us to become a net exporter of ideas and talent, and I think that’s something that's really happened.
"Our clients are now people who have come through Asia and moved into global roles, or are running global roles from Asia.
"The work coming from the region is recognised across a much broader swathe of markets in international award shows. I'd say Asia's position in the world has become more important within the ad industry. It's striking a note on the global stage as a fast-moving and entrepreneurial part of the world."
Robertson said Burge’s experience working with Thomas throughout his time in Asia meant that, "he knows what it takes to succeed in the region".
He also commended Potter and her "exceptional team" for having "built an incredible agency group over the past nine years". The agency has grown from a staff of 70 to more than 500 across Greater China.
Burge has done a commendable job running Singapore and more recently, Southeast Asia, agreed Thomas.
He said: "It shows he can operate across a group of markets effectively. He's a fantastic guy and knows our clients well."