He led the United Nations Environment Programme (UNEP) research groups in Kosovo, Afghanistan, Iraq, Liberia, Palestine and Sudan.

In the run-off on 5 February, he garnered more than one million votes (37.4 percent), coming second to National Coalition Party candidate, former Finance Minister Sauli Niinistö.

[11] In October 2018, the chairman of the Green League Touko Aalto announced that he was resigning from his post, citing depression and fatigue.

[17] In 2019, Haavisto called for the EU to intensify efforts on improving relations with Russia in the face of international conflict, transatlantic tensions and Brexit.

[17] In December 2020, the Parliament's Constitutional Law Committee found that Haavisto, who had pushed for Finnish children from the al-Hawl refugee camp in northern Syria to be brought to Finland quickly in late 2019, had broken Finnish law by trying to replace a foreign ministry official who refused to act after a difference of opinion over the exact repatriation proceedings.

Two Green League MPs published a dissenting opinion and advocated for a softer verdict, causing controversy in the Committee.

The complaint originated from a video recorded by a reporter for the gossip magazine Seiska, which included commentary that Haavisto had not stopped for two pedestrian crossings.

The prosecutor who reviewed the video evidence decided to not to press charges, citing construction work that had disabled the first crossing and created an unclear situation for the other.

Ethiopia's foreign ministry dismissed Haavisto's comments as "ludicrous" and a "hallucination of sorts or a lapse in memory of some kind.

"[29] Haavisto stated that Ethiopian criticism will have no effect on his work as envoy and referred to an upcoming UN human rights report on the situation.

After completing the matriculation examination of the upper secondary school,[37] Haavisto began studying social sciences at the University of Helsinki.

Although his homosexuality was an open secret during the 1990s, his sexual orientation wasn't public until 2002, when he entered in a registered partnership with Antonio Flores,[40] an Ecuadorian man born in Esmeraldas in 1978.