Matt Scheckner, the founder of Advertising Week Europe, which takes place this week, said he had to think about the possible impact on the UK economy when Britain leaves the European Union.
"To some degree, all bets are off when Article 50 is triggered," Scheckner said. "Five years from now, will Advertising Week Europe be in Amsterdam or Paris? I hope not.
"We’re ‘all in’ on London. But you hear what people are saying and uncertainty is not good for any business. It drives people nuts."
He said Theresa May, the prime minister, and Sadiq Khan, the mayor of London, have a "responsibility" to give businesses "certainty".
Scheckner, who founded Advertising Week in New York in 2004 and launched the European version in 2013, remains optimistic about London and compared it favourably with his hometown.
"London’s embrace of the creative industries is a differentiator between London and New York," he said.
"From an economic point of view, from a quality of life point of view, from an image point of view, the public and private sector leadership in London has recognised our industry matters.
"London is the epicentre of Europe. It’s the EMEA capital. Whether that remains the case [after Brexit] is to be seen."
Speakers at Advertising Week Europe 2017 include Khan, former US vice-president Al Gore, Interpublic chief executive Michael Roth, Facebook EMEA vice-president Nicola Mendelsohn and Google EMEA president Matt Brittin.
Brittin is expected to face questions about brands boycotting YouTube because of ads being placed next to inappropriate content.
Scheckner continues to expand the Advertising Week brand. He launched it in Asia last year and plans versions in Mexico and Australia.