He told CNN's Jake Tapper during a discussion Thursday at Harvard Kennedy School's Institute of Politics that Russia is behind a lot of fake news on Facebook.
"I think (fake news) was a huge problem," Mook said. "I think there's a lot of things that we need to examine coming out of this. You just named one of them. Congress has got to investigate what happened with Russia here. We cannot have foreign, and foreign aggressors I would argue, intervening in our elections."
The full interview with Mook and Kellyanne Conway, campaign manager for Donald Trump's successful presidential campaign, will air Sunday on CNN's "State of the Union."
Mook also blamed Breitbart News, which President-elect Trump's senior adviser Steve Bannon used to run, for "peddling stories" that are fake.
Bannon once referred to Breitbart News as "the platform for the alt-right," which critics say is rife with white nationalist, anti-Semitic and racist ideologies.
"And I'm not attacking (Bannon) personally, but they peddled a lot of stories on that website that are just false, they're just not true and that reinforced sexist, racist, anti-Semitic notions in people," Mook said. "You know, headlines that are shocking and insulting and shouldn't be part of our public discourse."
Conway countered the "biggest piece of fake news in this election was that Donald Trump couldn't win."
"And that was peddled probably for weeks and months before the campaign; definitely in the closing days," she said. "If you look at major newspapers and major cable station networks, Jake, it's unmistakable."
She continued, "America said that there's a difference between what may offend me and what absolutely affects me. And I, as a voter, am going to go that way and I'm going to vote according to what absolutely affects me."