After holding various administrative positions in Canberra, Millar was appointed the Ambassador for People Smuggling Issues in 2003. Serving until 2005, between February and April 2006 Millar acted as Head of Mission to the Permanent Mission to the UN in New York and was thereafter appointed to succeed Michael Smith as Permanent Representative of Australia to the United Nations Office in Geneva. In 2006, Millar was elected as President of the Anti-Personnel Mine Ban Convention's Seventh Meeting of the States Parties (7MSP), or Ottawa Treaty. Ambassador Millar was only the second woman to lead the treaty.