He worked for 22 years on The Guardian, ending his career in September 2018 as the newspaper's defence and intelligence correspondent.

MacAskill was involved in preparing the publication disclosures from Edward Snowden of the activities of the American National Security Agency (NSA).

[3] While based in the United States, he was involved in preparing the Edward Snowden revelations concerning the NSA for publication liaising with Snowden and his contact, Glenn Greenwald, who had brought the story to the attention of then GuardianUS editor Janine Gibson.

[2] As a result of his reporting on Global surveillance disclosures, he was named co-recipient of the 2013 George Polk Award.

The same reporting also contributed to the Pulitzer Prize for Public Service awarded jointly to The Guardian and The Washington Post in April 2014.