In 1967 he fought over the office of vice president of the National Union of Students against Jack Straw (later Labour Party MP and Secretary of State for Justice of the UK).

[4] Hagen has explained that he lost faith in the Conservative Party as an alternative to social democracy during the Per Borten cabinet (1965–1971), when taxes and the power of the state increased more than under Labour.

Hagen, in contrast, wanted to reduce the power of the state over individuals, and the political views of Anders Lange were because of this appealing to him.

[3] Hagen began his political career when, in 1973, he became a deputy representative in the Storting for the newly formed Anders Lange's Party.

Hagen has been regarded as the first postmodern politician in Norway by Gudleiv Forr, writing for the Norsk biografisk leksikon.

[3] His success has also been attributed to his leadership tactics, which included suspending and removing members of the party who deviated too much from his views.

Part of the decision not to run for re-election, which he had made in 2007, apart from reaching the age of pension, was because Hagen had felt that he had been put on the sideline after retiring as party leader.

[10] After ending his high-profile political career, he started working for the public relations agency Burson-Marsteller in 2009, where he became among the company's highest-paid lecturers.

[11] In March 2010 it was however speculated about his comeback into Norwegian politics, as "central Progress Party politicians" wanted him to run for mayor of Oslo.

[12][13] In September 2010, Hagen announced his candidacy for the office of mayor of Oslo for the 2011 local elections,[14] and he quit his engagement with Burson-Marsteller.

After the meeting, he published a five-page note, criticising Siv Jensen and citing his resignation to be because of the "treatment and humilitation" that he received from the "party leadership.

[9] In July 2016, Hagen endorsed Donald Trump for President of the United States, calling him "a man of the people" and comparing him to Ronald Reagan.

Largely because of those populist views on immigration, political opponents of the Progress Party have repeatedly resorted to physical assaults on Hagen.

He also said that if Israel lost the fight in the Middle East, Europe would "bow under to Islam" if Muslim fundamentalists get it as they want: "They have, in the same manner as Hitler, long ago made it clear that the long-term plan is to Islamify the world.

[29] Some days later, the ambassadors of Pakistan, Indonesia, Egypt and Morocco and the chargé d'affaires of Tunisia made an unusually-strong attack on Hagen in a letter in the newspaper Aftenposten.

In 2009, he received a "bridge builder award" from the Norwegian-Pakistani committee for the celebration of the Pakistani Independence Day in Norway, for his "strong engagement in integration politics.

According to Cappelen Damm, Hagen writes "openly about his strategic choices, about central political processes, conflicts and victories – and about how they formed him and the party."

He was also described as an "experienced and outspoken politician" who in his book was "hard-hitting, straight to the chase – and with continued willingness and ability to provoke.

In the book, he also compared what he saw as a similar naïvety in the modern Norwegian immigration policy, and Neville Chamberlain's failed negotiations with Adolf Hitler in the prelude to the Second World War.

[36] When discussing the aftermath of the Jyllands-Posten Muhammad cartoons controversy, he wrote that the management by the Norwegian government led freedom of speech to be "subordinated to the respect for the warlord, man of violence and female abuser Muhammad, who murdered and accepted rape as a conquest technique.

In the book, Hagen claimed that the policies used by the socialist parties were destroying the welfare state, by using poor solutions.