Brown joined Hotwire from Speeed in 2014, charged with meeting the client demand for non-traditional services such as design, insight, analytics and branding.
The announcement of Hotwire appointing its first chief client officer last month was accompanied by the news that Brown would relocate to New York to become group SVP for strategy and innovation.
However, staff were informed on Friday (3 November) in an internal email that Brown "has decided to leave Hotwire". He had informed the firm's global leadership team the week before.
"In true JB fashion, there'll be a fast and furious handover before he heads out on gardening leave on November 10th. There will be plenty of time to say our goodbyes as JB will be transitioning his responsibilities between then and Jan 19th, which will be his official last day," it continued.
CEO Barbara Bates said in the email: "John has made a huge impact on the business in so many ways and we'll miss his larger-than-life presence around the office. I'm personally disappointed in not getting the chance to work more closely with John in his new role and new home base. I'm sure whatever his future holds, he'll go after it full tilt as only John can do."
Brown declined to comment.
A spokesman told PRWeek: "We do plan to replace his role and will be looking at both internal and external candidates."
In September, another Hotwire UK board member Steven George Hilley departed to start his own agency, just days after two of its US staffers announced similar plans.