[2] From October 2013 to September 2014, she served as Under Secretary-General and Special Coordinator of the OPCW-UN Joint Mission in Syria to oversee the elimination of the country's chemical weapons program.

[8] She also received foreign relations training at the Clingendael Institute in The Hague, and studied at the French École nationale d'administration (ENA).

[10] In this capacity, she was the deputy to Helen Clark and oversaw UNDP's strategic external engagement, organization-wide communication and advocacy, as well as resource mobilization.

[2][citation needed] On 13 October 2013, United Nations Secretary-General Ban Ki-moon nominated Kaag to lead the OPCW-UN Joint Mission for the destruction of Syria's chemical weapons.

[13] Kaag led a team of one hundred experts who were responsible for ensuring the elimination of Syria's chemical weapon stockpiles before 30 June 2014.

[14] By the end of her term in September 2014, news media reported that Kaag was rumoured to succeed Lakhdar Brahimi as UN Special Envoy to Syria.

[2] On 1 December 2014, the UN Secretary-General Ban announced that Kaag would become the United Nations Special Coordinator for Lebanon (UNSCOL), succeeding Sir Derek Plumbly.

[15] After the resignation of Foreign Minister Halbe Zijlstra on 13 February 2018, she was appointed ad interim to succeed him while conserving her other cabinet position.

From 2018 to 2022, Kaag served on the joint World Bank–WHO Global Preparedness Monitoring Board (GPMB), co-chaired by Elhadj As Sy and Gro Harlem Brundtland.

[18][19] In 2019, she joined the World Economic Forum High-Level Group on Humanitarian Investing, co-chaired by Børge Brende, Kristalina Georgieva and Peter Maurer.

[23] On 21 June 2020, Kaag announced her candidacy for lijsttrekkerschap of the Democrats 66 for the 2021 general election, with the ambition of becoming the Netherlands' first female prime minister.

[37] During a lecture in the Rode Hoed debating centre in September of the same year, she voiced fierce criticism of the political culture that had arisen partly under Mark Rutte's leadership.

A majority reproached her for the late evacuation of Dutch citizens and local civilian personnel from Afghanistan amid the 2021 Taliban offensive and Fall of Kabul.

Days before, a candidate of the Jezus Leeft party had stood outside of Kaag's home with a burning torch, and security measures were subsequently raised.

[6] Trouw profiled Kaag as a typical frugal Dutch finance minister who forced spending cuts following years of economic stimulus programs during the COVID-19 pandemic.

[46] On 12 July 2023, five days after the collapse of the fourth Rutte cabinet, Kaag announced in an interview in Trouw that she would not lead D66 into the 2023 general election, due to the impact of the "hate, intimidation and threats" her family was receiving.

Its article referenced public perceptions of Kaag as an elitist politician as well as PVV leader Geert Wilders calling her a witch in 2021.

[51] When Tor Wennesland left the United Nations, Kaag took over his position as Special Coordinator for the Middle East Peace Process and as Personal Representative of the Secretary-General to the Palestine Liberation Organization and the Palestinian Authority on 17 January 2025, days after Israel and Hamas agreed to a three-phase ceasefire deal.