[2] From September 1994 to January 2001 she was head of the Press and Public Information Team at the Organization for Security and Co-operation in Europe in Vienna.

In January 2001 Fleming took over as Spokesperson and Head of the Media and Outreach at the International Atomic Energy Agency in Vienna, a post she held until June 2009.

[3][4] At UNHCR, she led campaigns, social media engagement[5] and a multimedia news service[6] covering stories designed to generate empathy and stir action for refugees.

In 2017 she wrote A Hope More Powerful Than the Sea book about Syrian refugee Doaa Al Zamel's experiences leading up to and during the 2014 Malta migrant shipwreck.

On September 1, 2019, she was appointed an Under Secretary-General of the United Nations, taking over from predecessor Alison Smale as head of the UN Department of Global Communications.