She was re-elected as member of parliament in April 2023 but resigned to become a strategic adviser on political leaders' reform programmes in the Tony Blair Institute in September 2023.

Following Antti Rinne's resignation in the wake of the postal strike controversy, Marin was selected as prime minister on 8 December 2019.

[14][28][25] Upon her confirmation by the Finnish parliament at the age of 34, she became Finland's youngest-ever Prime Minister,[29][30] making her the then youngest serving head of government until Sebastian Kurz regained that description in January 2020.

[33] When Swedish Prime Minister Stefan Löfven could not attend a European Council meeting in October 2020 because of his mother's funeral, Marin stepped in to represent Sweden.

On 25 February, a Russian Foreign Ministry spokesperson threatened Finland and Sweden with "military and political consequences" if they attempted to join NATO, which neither were then actively seeking.

[44] On 26 May 2022, Marin went to Kyiv at the invitation of Ukrainian Prime Minister Denys Shmyhal,[45] where she met President Volodymyr Zelenskyy and visited the war-torn cities of Irpin and Bucha.

On 26 May 2022, Marin also signed a bilateral framework agreement on the rebuilding of Ukraine's education with Ukrainian Prime Minister Denys Shmyhal.

[46] On 31 May, she welcomed a deal agreed by all EU leaders to ban more than 90% of Russian oil imports by the end of the year, voting for it in the European Council.

[52] Marin re-iterated her call for a fighter aircraft discussion as "the next big question" after main battle tanks, stating that Ukraine's defense against Russia requires heavier weaponry.

[53] Sanna Marin's attitude, as the vice chairman of SDP, was negative towards Finland joining NATO and pro-NATO statement by Finnish Defence Minister Carl Haglund after Russia conquered Crimea in 2014.

[56][57] During her year address at the beginning of 2022, in response to Russian aggression at the Russo-Ukrainian border, Marin stated that Finland had the right to join NATO if it wanted to, and should consider this option.

[60] On 15 May 2022, Niinistö and Marin announced that Finland would apply for NATO membership, and on 17 May the Finnish parliament approved the proposal in a vote of 188–8.

[64] In October 2020, an interview of Marin in the Finnish lifestyle magazine Trendi about her job and its expectations was illustrated by a photograph in which she wore a low-cut trouser suit with no shirt underneath.

[65] On 25 May 2021, Finnish media reported that Marin and her family were spending about €300 per month on groceries with public funds as a part of the Prime Minister's tax-free housing benefits in the official residence, Kesäranta.

The legality of the customary arrangement that had been in place for decades was questioned since the rules on the residential benefits did not explicitly mention food provisions.

[69] Marin and her family had used around €14,363.20 on catering services in the form of breakfast provisions and cold evening meals in the Prime Minister's official residence between January 2020 and May 2021, equivalent to €845 per month.

[71] It turned out that civil servants in the Prime Minister's Office (VNK) handled the payments and Marin was not informed of the running costs of the residential perk.

[85] There was further controversy after an image of Marin's friends from a party in the prime minister's official residence were leaked depicting two topless women kissing with a "Finland" sign covering their breasts.

[90] On 24 February 2023, the constitutional law committee voted 9–7 to suspend work on the bill, preventing the legislation from being passed before the 2023 Finnish parliamentary election.

[100] In August 2023, Marin asked Jutta Urpilainen to be Social Democratic Party's candidate for the 2024 presidential election of Finland.

[101] On 7 September 2023, Marin announced her intention to resign as an MP and take up a position as a strategic advisor at the Tony Blair Institute for Global Change.

[102][103] On 28 September 2023, it was reported that she had signed with talent agency Range Media Partners for film, TV, audio and brand collaborations.

[104] In January 2024, Marin started working in a new steering committee International Task Force on Security and Euro-Atlantic Integration of Ukraine.

According to a statement released by the Ukrainian president's office, the steering committee's primary objective is to develop a strategy for Ukraine's closer engagement with the Euro-Atlantic security area.

The group is led by its founders Andriy Yermak, Chief of Staff of the President of Ukraine and former Nato Secretary-General Anders Fogh Rasmussen.

In addition to them, 15 former European and North American heads of state, diplomats and officers (including Boris Johnson and Hillary Clinton) are members of the group.

[118] Marin was selected for the cover of Time magazine's "Time100 Next" theme issue, which showcases one hundred influential leaders from around the world.

[125] As Finland’s prime minister Marin was awarded a Doctor of Humane Letters, honoris causa at New York University.