Mulet is going to replace the outgoing Chef de Cabinet Susana Malcorra of Argentina, who has been named Argentina's incoming foreign minister.
Ban has expressed his "deepest" gratitude for Malcorra's "tireless support, outstanding leadership and admirable dedication."
Mulet has been assistant secretary-general for peacekeeping operations since 2007, with a term from 2010 to 2011 as special representative of the Secretary-General and head of the United Nations Stabilization Mission in Haiti (MINUSTAH).
Chef de cabinet is the head of an office in the UN system appointed by the UN chief for his personal cabinet. The position's rank and responsibilities are equivalent to a chief of staff.