Petrie was educated at the American College in Paris, graduating BA in International Relations, and at INSEAD, where he gained the MBA degree.
After a year he returned to the field, in this case the Middle East, and while based in Gaza from 1996 to 1998 he assumed the responsibilities of Special Assistant to the Commissioner General of UNRWA.
In early December 2011, he undertook a mission into the area controlled by the Karen National Liberation Army 5th Brigade to assess the willingness of the armed resistance to engage in ceasefire talks with the Myanmar government.
In March 2012, Petrie was designated by the UN Secretary General to lead an internal review of the UN's actions in Sri Lanka during the last phase of the conflict.
Petrie was appointed Officer of the Order of the British Empire (OBE) in the 2014 New Year Honours for services to international peace, security, and human rights.