Before entering national politics, he was best known for criticising multiculturalism and Finland's immigration policies in his online blog, Scripta.

He later expressed regret at his decision, calling the choice a "stupid political protest", and voicing support for the present conscription system.

[18] In the parliament he was made chairman of the Administration Committee,[19] which deals with immigration affairs among other matters.

[28][29] His selection resulted in a political crisis in Finland, as the leaders of the two other governing coalition parties, Prime Minister Juha Sipilä of Centre and Finance Minister Petteri Orpo of the NCP, stated they would not co-operate with the Finns Party led by Halla-aho.

[30][31] The two leaders argued that their decision was based on value differences between them and Halla-aho's policies, and that the Sipilä Cabinet would duly be dissolved.

[32] On 13 June, however, twenty Finns MPs, including former leader Soini, broke away from the parliamentary group and founded New Alternative.

Sipilä announced that his cabinet would continue working with New Alternative (later called Blue Reform), thus securing a parliamentary majority, and leaving the remaining Finns MPs in opposition.

[9] In June 2021, Halla-aho informed that he is not running for re-election as chairman of party, but will continue in parliament and municipality.

[43] Halla-aho voices support for the welfare state, and places himself on the left side of the political spectrum, in matters of economic policy.

[44] Halla-aho maintains a blog titled Scripta which states that it treats issues such as "immigration, multiculturalism, tolerance, racism, freedom of speech and political correctness".

Referring to his own works, he has affirmed that criticising "totalitarian fascist ideologies like political Islam" should not be considered racism and that facts cannot be criminalised.

[44][47] Halla-aho has been accused of racism by academics and members of the Finnish government,[51][52] and has been connected to the counter-jihad movement,[53][54][55] having previously been a contributor to the blog Gates of Vienna.

[44] On 19 April 2022, in the aftermath of Russian invasion of Ukraine when German Bundestag member Tino Chrupalla from right-wing populist AfD praised Chancellor Olaf Scholz's decision not to send heavy weapons to Ukraine and demanded an end to sanctions against Russia and for peace negotiations to start, Halla-aho responded on X (formerly Twitter) in German "I find it impossible to understand how a nationalist party can support or justify imperialism.

"[57] (Ich finde es unmöglich zu verstehen, wie eine nazionalistische Partei Imperialismus unterstützen oder rechtfertigen kann.)

[58][59] On 27 March 2009, the Helsinki District Court ordered Halla-aho to stand for trial on charges of ethnic agitation and breach of the sanctity of religion.

The text discussed the limits of free speech and intentionally made some controversial claims in order to check the response of judiciary.

[60] In another text, he asked if it could be stated that robbing passersby and living on taxpayers' expense are cultural and possibly genetic characteristics of Somalis.

[52] The text was originally a response to a Finnish columnist of the newspaper Kaleva, who had written that drinking excessively and killing when drunk were cultural and possibly genetic characteristics of Finns and was intended to underline the hypocrisy of it being possible to be prosecuted for one of those essentially similar statements but not for the other.

[61] On 8 September 2009, the District Court convicted Halla-aho of disturbing religious worship, and ordered him to pay a fine of 330 euros.

[64] In a sentence given on the 8 of June 2012, the Supreme Court found Halla-aho guilty of both disturbing religious worship and of ethnic agitation and increased his fines accordingly to 400 euros.