Allen is an internal promotion, and started at Fonterra as a graduate in 2008.
The appointment is for two roles: as Marsh McLennan NZ chief executive, and Marsh NZ president.
Tracey Taylor will commence her role on October 2, 2023.
The former Cabinet minister will be its new commercial director, based in Wellington.
Tim Grafton will leave the role next April.
Former IBM managing director and NZTech board member Mike Smith takes up the role.
Priority One is the Western Bay of Plenty’s economic development agency.
Scott Weenink has stepped down as chair to focus on his new role as CEO of New Zealand Cricket.
Experienced director Jim Sybertsma will join the board from October.
Wellington Zoo chief executive, Karen Fifield MNZM is the first New Zealander to hold the global post.