Nick Brien, chief executive of Dentsu Aegis Network Americas, is to quit after two-and-a-half years—the latest in an exodus across the Japanese-owned agency group.
Jacki Kelley, president and chief client officer at Dentsu Aegis US, will succeed Brien on 1 January 2020.
Brien recruited Kelley, a former deputy chief operating officer at Bloomberg and ex-chief executive of IPG Mediabrands in North America, earlier this year.
Campaign has been aware since mid-November about speculation that Brien could be heading out.
Since Brien joined in June 2017, Dentsu Aegis has won some significant business, including LVMH and Intel, expanded its relationship with Procter & Gamble and retained Microsoft.
Dentsu Aegis Americas has grown 2.7% in the first nine months of the year, with the third quarter up 5%.
By contrast, as a whole Dentsu Aegis' revenues have gone into reverse, sliding 0.2% in the nine-month period.
Brien is shortlisted for best agency head at the Campaign US Agency of the Year Awards, which take place in New York in January 2020.
Tim Andree, long-serving chairman of Dentsu Aegis, took over Buhlmann’s duties and has been shaking up the group amid a wider move by Dentsu Inc to bring its Japanese and international operations together as "one Dentsu".
Other senior figures to depart from Dentsu Aegis this year include Nigel Morris, global chief strategy and innovation officer; Susana Tsui, chief executive of China; Dick van Motman, global chief executive of content and creative; and Nick Waters, chairman of Dentsu Aegis UK and Ireland.
Andree said: "During Nick’s time as DAN Americas CEO, he has made a significant contribution to the business’ turnaround. Nick has attracted key talent to Dentsu Aegis, including Jacki, led many impressive winning pitches and, as importantly, reignited the M&A momentum across the region.
"As DAN US president, Jacki Kelley brings the right leadership acumen, client management skills and commercial experience as well as operational, organisational and strategic expertise. Jacki is Nick’s ideal successor to take the Americas business forward."
Brien said: "Jacki has been a recognised and admired leader in this industry for over 25 years. Since bringing her into DAN earlier this year as US president, she has made tremendous progress in architecting our client and brand solutions strategies across all capabilities, while also leading the US creative line of business."
He added: "She is absolutely the right person to lead Dentsu into 2020 and beyond."
Kelley said she was "honoured" to take the job and is confident that "2020 will be another pivotal year for growth" for the Americas.
There have been rumours that Brien and Andree were not close and Dentsu Aegis ran into a legal row earlier this year over its involvement in a costly, 50th-anniversary Woodstock festival that was subsequently cancelled.
A spokesman for Dentsu Aegis offered no comment on that.