Bannon met with Robert Mueller for about 20 hours, according to the NBC report, which cited two sources familiar with the proceedings.
Bannon, who also ran Trump's campaign during the final stretch of the 2016 presidential race, left the White House in August. He returned to his position heading up the right-wing online news outlet Breitbart afterward, but he departed the organization after his quotes in an explosive tell-all book went public earlier this year.
In the book, "Fire and Fury," author Michael Wolff quotes Bannon from his time in the White House as insulting Trump's adult children Ivanka and Donald Jr.
Last month, Bannon reportedly struck a deal with Mueller's team after he was subpoenaed, sparing him from testifying before a grand jury.
Mueller is investigating Russian meddling in the 2016 election and any potential ties between the Trump campaign and the Kremlin. Trump has repeatedly denied colluding with Russia in the election.