Watson, currently the CSO at BBH New York, will report to Neil Munn, the global chief executive, and work alongside Pelle Sjoenell, who was appointed worldwide chief creative officer in January.
Emma Cookson, the former chairman of BBH NY, was the last person to hold the global CSO position, but she left in 2015.
Watson said: "For me, BBH isn’t a job, it’s more like a calling. BBH is founded on the power of having the three different voices of creative, account management and strategy shaping the work.
"I am immensely proud to be part of the team taking this into the next generation. "
Watson first joined BBH 14 years ago, starting at BBH London as a planner on brands including Johnnie Walker, Vodafone and Levi’s. She was then head of planning at DDB London before returning to BBH in 2011 as CSO of the agency’s New York office, a role she will continue to hold.
Sjoenell said: "I cannot think of a better partner in crime than Sarah Watson. I have always looked up to her professionally, intellectually and as a leader. She is the kind of person I want to impress."
As well as producing award-winning work for clients including PlayStation, Netflix, Seamless, Johnnie Walker and Cole Haan, Watson has served on juries for awards including Effies and the ADC. She is also focused on improving diversity in the industry.
Watson was responsible for spearheading BBH NY’s "Griffin Farley Search for Beautiful Minds" – a three-day mentorship program culminating in a talent showcase in honour of a late colleague.
Earlier this year, Watson was announced as the first ambassador for Cannes Lions’ "See It Be It" programme, an initiative which aims to develop high-potential creative women and bring them to the attention of the industry. She has also been a Trustee of Fawcett, the UK's leading gender equality campaigning organisation, and works with similar groups in the US.
Yesterday, Campaign revealed that Jonathan Bottomley, one of the joint chief strategy officers at BBH London was leaving to relocate to New York with his family.