Diplomatic service

During her time in the diplomatic service, she served in Spain, Hungary, and Belgium. She also served as the deputy chief of mission of the United States Mission to the Organization for Security and Cooperation in Europe, where she expressed dismay at Russian aggression against Ukraine.^[3] After that, she became deputy chief of mission in Greece from 2017 to 2019.^[4][5]

From 2007 through 2010, she was a public affairs advisor to the United States Mission to the North Atlantic Treaty Organization. She has previously represented U.S. interests in Bolivia, Hungary, Spain, and Turkey.^[6]

Ambassador to North Macedonia

She was originally nominated to North Macedonia in 2018,^[7] and re-nominated in 2019, both before and after the nation changed its name.^[8] Byrnes was appointed to replace Ambassador Jess Baily.^{[6][9][10][11][12]}

A career diplomat, she had served as the deputy ambassador in Athens, and was the designated representative of the U.S. mission to the Organization for Security and Cooperation in Europe in Vienna.^[11][6] Addressing the United States Senate during her confirmation hearings, she averred her continuing commitment "to strengthen cooperation with (North) Macedonia as a strategic partner." She highlighted that the Prespes agreement surmounted an historic hurdle, so its implementation is regionally important.^[11] Upon her appointment, acting Assistant Secretary of State Philip Reeker saw her appointment as a harbinger of "a new era of bilateral cooperation, as friends and allies of the Republic of Macedonia."^[6] She brought twenty-four years experience as a diplomat before being appointed to the post.^[5]

Fulfilling its responsibility under the Foreign Service Act, Section 304(a)(4), the Senate Foreign Relations Committee noted that she has a well-documented path of competency in extremely demanding positions.^{[upper-alpha 1]}