[9] After his arrest, Kristinn Hrafnsson introduced him to Julian Assange, the editor and founder of WikiLeaks, and he worked as a volunteer for the organization between 2010 and 2011.

[10] In 2011, Thordarson contacted the FBI and offered to become an informant, turning over numerous internal WikiLeaks documents and hard drives in the process.

[14] Thordarson was a key witness whose testimony was crucial to the US case, according to numerous news organisations, including the German public broadcaster Deutsche Welle, The Hill, Der Spiegel and The Intercept.

[25] The information published by Icelandic news media obtained from Thordarson also showed that one of the country's biggest football stars, Eiður Guðjohnsen, was deeply indebted and almost bankrupt.

Other names in the documents leaked by Thordarson included information about Birkir Kristinsson, who had recently been convicted of economic crimes while working for Glitnir bank.

"[23] Birgitta Jónsdóttir, a former WikiLeaks volunteer and member of the Icelandic parliament who worked on Collateral Murder, described Julian Assange's relationship with Thordarson as like "Batman and Robin.

"[23] Several of WikiLeaks' core volunteers said that Siggi was a dangerous liability, prone to indiscretions and lies and Jonsdottir warned Assange not to trust Thordarson.

[22][23] Assange asked Thordarson to write psychological profiles of core WikiLeaks members and to install hidden cameras to spy on guests in Ellingham Hall.

[23] According to an indictment against Assange, he and Thordarson made a "joint attempt" to decrypt a file stolen from an Icelandic bank.

[3][11][41] David Kushner reported that Thordarson provided Rolling Stone with over 1 terabyte of data about WikiLeaks, including thousands of pages of chat logs, videos, tapped phone calls, government documents.

Kushner said that either Thordarson was the real deal or this was the biggest and most elaborate lie in the digital age, and that Assange's affidavit validated the importance of Siggi’s documents.

"[23] Thordarson embezzled $50,000 from a WikiLeaks online store that sold T-shirts, the money was paid into his own bank account and the official reason why he was fired.

[5][3][39] During his period at WikiLeaks, it has also been reported that Thordarson ordered attacks on Icelandic governmental infrastructures such as the servers hosting the Ministry's websites stjornarradid.is and landsnet.is.

It has also been reported that Thordarson ordered Hector Monsegur (Sabu) and his team to attack Icelandic State Police servers.

[3] Morgunblaðið, Iceland's largest newspaper published on the front page on 31 January 2011 that a local reporter for the paper DV was suspected of obtaining the information from Thordarson.

[53][54] Some Icelandic papers have connected the phone call recordings that WikiLeaks allegedly got to "Spy Computer" scandal.

[56][57][58][59] In a June 2021 interview with Icelandic media Stundin, Thordarson said that Assange did not ask him to hack or record phone calls of the MPs.

[39][22] The day after the meeting with the embassy official the FBI sent a private jet with eight federal agents and a prosecutor to question Thordarson.

In 2012, he met with the federal agents on multiple occasions, and was flown to Copenhagen where Thordarson was provided a room in a luxury hotel.

Thordarson met with the agents there and handed over several hard drives he had copied from Assange and core WikiLeaks members.

[3][62] Thordarson said that in February 2013, he told Kristinn Hrafnsson, Ingi Ingason and a third WikiLeaks staff member that he was an FBI informant and gave them the 3TB trove.

[63][64] The then-Minister of the Interior Ögmundur Jónasson said in Parliament that Thordarson was young and the FBI meant him to be a "spy" within the WikiLeaks organization.

Several news organisations, including The Hill, Deutsche Welle, Der Spiegel, Berliner Zeitung and The Intercept described Thordarson as being a chief or key witness in the case.

[15][16][17][18][19] Deutsche Welle said "the key witness, Icelandic national Sigurd Ingi Thordarson, had admitted to fabricating incriminating testimony against Assange in return for immunity from prosecution".

[16] Over ten days after the Stundin article, The Washington Post said Thordarson's testimony was not used as the basis for charges but for information on Assange's contact with Chelsea Manning.

In 2014, Thordarson was ordered to pay WikiLeaks 7 million ISK (roughly $55,000) as well as being sentenced to prison for 2 years for embezzlement and financial fraud.

A psychiatric evaluation ruled that Thordarson was of sound mind, but that he had an antisocial personality disorder[72] and was incapable of feeling remorse for his actions.

[78] The film The Fifth Estate (2013), with Benedict Cumberbatch as Assange, features a character based on Thordarson's played by Jamie Blackley.