From 1992 to 1995 she worked in Washington, D.C., as Private Secretary to the British Ambassador to the United States, Robin Renwick.

[9] From 2012 to 2015 Pierce was posted to Geneva as the UK's Permanent Representative to the United Nations and other international organisations there.

[14] On 7 November 2024, following the 2024 United States presidential election, the Chancellor of the Duchy of Lancaster, Pat McFadden, announced that Pierce was doing "an excellent job" as ambassador, with the "full confidence of the British government", and would remain in Washington until after Donald Trump's inauguration in January 2025.

[15] Pierce is considered broadly popular across the political spectrum in the United States, with senior Republican campaign adviser Chris LaCivita, the co-campaign manager during Donald Trump's victorious 2024 presidential campaign, describing her as "professional universally-respected" and bemoaning her replacement.

[16] Known for her flamboyance and colourful outfits, Pierce has made regular appearances on Fox News and other US media.

[5] On 20 December 2024 Prime Minister Sir Keir Starmer announced the appointment of Lord Mandelson to be the next British Ambassador to the United States of America.