He served as the Speaker of the Parliament of Finland from 2007 to 2011 and has been the honorary president of the European People's Party (EPP) since 2002.

Niinistö was the NCP candidate in the 2012 presidential election, defeating Pekka Haavisto of the Green League (VIHR) with 62.6% of the vote in the decisive second round.

Niinistö was urged by his party to stand as a candidate in the 2000 presidential election, but he refused the candidacy for completely related to his private life reasons.

After the end of his term as a cabinet minister in 2003, Niinistö became vice president at the European Investment Bank.

[18] After becoming the President, Niinistö pledged to establish a special task force aiming at preventing alienation among the country's youth and expressed concern about the problems of sparsely populated rural areas.

[23] By 2022, Niinistö was estimated to have an approval rating of 92 percent, but rejected efforts by a grassroots movement to allow him to run for a third term.

He discussed ice hockey and business, but not human rights issues or the selling of Russian military equipment to Syria and its transport through Finland.

He also stated that "Russia understands that the conflict in Ukraine has generated debate in Finland over this country's own security policy.

Finland accepts that Russia is working to find a solution to the acute conflict in Ukraine, but it needs to do more.

He knows that the answer is negative: nobody needs this"; Lavrov added "President Niinistö realizes that what happened in Ukraine is impossible in Finland.

"[31] Niinistö met with Iranian Supreme Leader Ali Khamenei on 26 October 2016 in Tehran, Iran.

They discussed closer economic cooperation, human rights in Iran, the situation in the Middle East and the threat of terrorism.

[36] In an interview with The Washington Post in January 2022, Niinistö stated that he could not speak to Putin's intentions regarding the frozen relations between Russia and the West, but cited what he described as Finnish "wisdom" on experiences in dealing with Moscow.

"[37] When he was asked to shed light on his thoughts about Putin after the Russian invasion of Ukraine on 24 February 2022, Niinistö replied: "The masks have now been taken off, showing only the face of war.

"[38] On 4 March 2022, Niinistö visited Washington, D.C., to meet President Joe Biden and number of other US politicians and security personnel.

[39] On 12 May 2022, ten weeks after the beginning of the Russian invasion of Ukraine, Niinistö and prime minister Sanna Marin advocated in a joint statement for a 'NATO membership without delay'.

[40] Niinistö also made his last call with Putin that month, telling him that Finland's decision to join NATO was the Russian leader's "own doing.

[42] On 21 May 2022, Turkish President Recep Tayyip Erdoğan told Niinistö that "overlooking threatening terrorist organisations that pose a threat to a NATO member is not in the spirit of the alliance.

"[43] On the same day, Niinistö, after a telephone conversation with Erdoğan, announced that they were ready for dialogue with Turkey regarding Finland's membership in NATO and that they always condemned terrorism.

[46] During Niinistö's presidency, neighbouring military cooperation ties between Finland and Sweden were further strengthened, culminating in joint, rather than separate, NATO accession bids initiated by both countries.

[48] Finland and Sweden agreed to a deal with the Government of Turkey that would allow the countries to join NATO on 28 June 2022, in the wake of the veto threat.

[50] As President of Finland, Niinistö held the power of pardoning for individual criminal sentences and related sanctions.

[52] Niinistö had explained that he was in good agreement with the policy of the Supreme Court of Finland, which had always processed the case first before the President saw it.

[54] In 2025, Niinistö criticised European countries for failing to playing a central role in engaging Russia to end its war on Ukraine.

Niinistö wrote about the time after the death of his first wife in his book Viiden vuoden yksinäisyys (translation: "Five years of loneliness").

Karpela's Centre Party was in opposition and Niinistö was considered the second-most influential man in government.