"This meeting was ill-advised," Christie said on CNN's "State of the Union." "This is not something that should have happened. ... Everybody in retrospect knew this was a bad idea."
Christie said it was not yet clear whether Trump had known about the meeting, which took place in Trump Tower and included his son-in-law Jared Kushner and then-campaign chairman Paul Manafort.
The Republican governor also said it was unclear what role Trump played in crafting Trump Jr.'s initial response
to news reports about the meeting, which Trump Jr. first said was "primarily" about adoption and its relation to US sanctions on Russia under the Magnitsky Act.
Within days of that first statement, as more details about the meeting became public, Trump Jr. posted emails on Twitter that showed he took the meeting in hopes of gaining damaging information on Hillary Clinton that would be provided as part of the Russian government's support for his father.
The details of Trump Jr.'s meeting were first reported by The New York Times.
White House press secretary Sarah Huckabee Sanders said Tuesday that the President did weigh in
on the initial statement Trump Jr. released July 8 about the Trump Tower meeting. Sanders called the matter "of no consequence."
"The statement that Don Jr. issued is true," Sanders said. "There's no inaccuracy in the statement. The President weighed in as any father would, based on the limited information that he had."
On Sunday, Christie advised everyone to wait for Mueller's investigation to go through that meeting and its aftermath before drawing conclusions about it.
"Let's have the facts come out," Christie said.
Scope of investigation
The Wall Street Journal and Reuters reported Thursday that Mueller had convened a grand jury as part of the special counsel probe, and a source told CNN
that he issued subpoenas for documents and testimony related to the Trump Jr. meeting.
Christie said Sunday that media coverage of Mueller's grand jury activities has been over-hyped, calling the actions a natural part of the investigative process.
"Let him do his work," Christie said.
However, Democratic Rep. Adam Schiff said in a separate interview on the program that while the move was "not unexpected," it was significant because it showed the investigation had reason to move forward.
"That wouldn't be taking place if there was really no evidence, no evidentiary basis to move forward," Schiff said of the grand jury news.
Trump has repeatedly railed against the investigation as a "witch hunt" and denied any wrongdoing.
Christie said Mueller, who was director of the FBI when Christie was a US attorney, is a "good man" who would try not to overstep his mandate -- although that was always a concern with special counsels, he added, because they might try to produce results to justify their appointments.
"I trust that he'll be very careful to try not to go on a fishing expedition," Christie said. "There's always a temptation to do that. I hope that's not what he does."
Deputy Attorney General Rod Rosenstein, who named Mueller special counsel in May, said on "Fox News Sunday" that the Justice Department does not do "fishing expeditions."
"Bob Mueller understands and I understand the specific scope of the investigation," Rosenstein said.
Rosenstein said if Mueller discovered evidence of a crime that's within the scope of the investigation, then he could pursue it, and if Mueller found "something outside of the scope," he would need to come to Rosenstein for permission to expand the investigation.