Before his arrest in 1984, he was successively a journalist, a junior Norwegian Labour Party politician and a medium-level official of the Ministry of Foreign Affairs in Norway, while secretly working for the KGB.

Treholt was found to possess a secret bank account in Switzerland with a substantial illicit amount.

In 2010, his case took a new twist when Geir Selvik Malthe-Sørenssen, who was later revealed to be a con artist and sentenced to prison himself, claimed that Arne Treholt had been the victim of a conspiracy.

Treholt was occasionally active in public debate, and he was accused by Norwegian media of promoting Russian propaganda.

[6] Aftenposten's foreign affairs editor Kjell Dragnes wrote that Treholt and Diesen promoted Russian propaganda.

On 20 January 1984, he was arrested by Ørnulf Tofte, head of counterintelligence, at Oslo Airport on his way to Vienna to meet with KGB officers.

[citation needed] Furthermore, Treholt during the trial admitted to having received "expenses" from the KGB, but claimed that it could "only" have been 26,000 or 27,000 USD.

Treholt was convicted and sentenced for passing classified material to KGB in the period 1974–1983 and to the Iraqi Intelligence Service 1981–1983.

The sentence also encompassed handing over secrets obtained at the Norwegian Joint Staff College where he was enrolled with authorization from the non-socialist coalition government.

[13] Malthe-Sørenssen was publicly exposed as a fraudster in 2016, and his alleged source was revealed by Verdens Gang to be a used car salesman and convicted murderer and con artist with no connection to the Treholt case, who was paid by Malthe-Sørenssen to impersonate a police security service employee.

[3] After the verdict, PST-director Kristiansen, who had denied re-opening the case in 2008, demanded an apology from Treholt's supporters.

In 2014 Treholt's lawyer Harald Stabell claimed to have been tipped off by an employee of Police Security Service that his law office had been under audio surveillance in 2010 and 2011.

In March 2006, media said[20] that Treholt had been admitted to a hospital in Cyprus and was in a stable but critical condition, and in a coma, possibly suffering from blood poisoning.