"It's concerning that Donald Trump was potentially sharing information that he learned in his briefing," Mook said on "Good Morning America."
Clinton and Trump participated in a wide-ranging forum Wednesday night hosted by NBC/MSNBC, in which Trump seemed to discuss some of what he had learned in his two classified intelligence briefings.
"What I did learn is that our leadership, Barack Obama, did not follow ... what our experts said to do," Trump said Wednesday night during the forum. "In almost every instance, I could tell -- I'm pretty good with the body language -- they were not happy. Our leaders did not follow what they were recommending."
Trump did not get into specifics about what he learned in the briefings, one Aug. 17 and the second Sept. 2.
Clinton's campaign manager went on to attack Trump this morning on “GMA,” arguing that Trump doesn't have a strategy to combat ISIS.
"What was really scary was he keeps talking about the secret plan that he has to defeat ISIS," Mook said. "What was clear last night, there's really no plan at all. That's what's scary."
Trump said Wednesday night, as he has many times before, that he does not want to share the details of his strategy because he does not "want to broadcast to the enemy exactly what my plan is."
Mook said today: "Last night, it was clear there just isn't a plan and he's trying to hide from that and he lashed out at our nation's generals when he was asked for details.”
Trump also said Wednesday night he thinks that under the "leadership of Barack Obama and Hillary Clinton, the generals have been reduced to rubble" and, in the past, he has claimed to "know more about ISIS than the generals."
"This is what is so incredible and challenging for the Republicans. The fact that the only criticism they could have for Hillary Clinton is that she didn't smile enough at a very serious forum on our veterans, on our national security and on the job of commander in chief just tells you everything you need to know," Mook said.
"Donald Trump simply didn't come prepared and he's not going to be prepared to serve as president and that was abundantly clear."