Millar and Gronntun have shared responsibility for global operations since they were promoted in October last year.
Millar will now take responsibility for the US, in addition to the global client, creativity and innovation agendas.
He led H+K’s UK operation for more than ten years and since 2014 driven the agency’s global creative agenda including the formation of the Global Centre of Creative Strategy and the Shanghai Addition in China.
It is unusual for a UK comms executive to make the move across the Atlantic for an agency leadership role.
Millar said: "I have gotten to know colleagues and clients in the US over the past year and I have been impressed by the depth and diversity of our work. We have brought in new talent, and built new capabilities that include a planning function, behavioral science expertise and the establishment of The Studio in New York."
Gronntun will lead its international portfolio taking responsibility for the major markets of UK, Germany, Canada and Greater China as well as the Asia, LatAm, METIA, Continental Europe and Nordics clusters.
He will also continue to oversee the global operations agenda.
Gronntun has led H+K’s largest region - Europe, Middle East and Africa - since 2014 and has worked with some of the agency’s top clients, including Equinor (formerly known as Statoil).
"I look forward to continue driving the momentum of our International portfolio," said Gronntun. "The diversity and strength of our international operations have always set us apart as one of the strongest global public relations agencies."
Both will continue to report directly to AnnaMaria DeSalva, who was named as global chairman and CEO of the firm in May this year after Jack Martin stepped down.
She said the agency was laying the foundations for the next era of growth and achievement.
"With the establishment of this leadership structure we are doubling down on our growth imperatives for major markets and clusters while positioning key functions, sectors, and capabilities to drive our progress," added DeSalva.
In addition, finance will be led by Andy Scharf, HR and talent by Kate Augustine, and business development and marketing by Sam Lythgoe.
Global counsel Meredith Marks will now also report directly to DeSalva, as will global sector leads.