Barr was asked repeatedly about a memo that he wrote to the Justice Department, in which he weighed in with his thoughts about questions he had about the scope of the Mueller investigation.
"Bob will be allowed to finish his work," Barr said flatly, pledging as much transparency as possible to release the details of Mueller's findings.
"The country needs a credible resolution to these issues," Barr said.
Barr also revealed that he met with President Trump in 2017, when the White House was looking for lawyers to defend the President with regards to the Special Counsel investigation.
"How well do you know Bob Mueller?" Barr recalled being asked, describing the meeting as a short one, to which there was no follow up, until the President moved to select him as the Attorney General, following the departure of Sessions.
As Democrats pressed Barr about his memo on the Russia probe, Barr said Mueller could only be fired for 'good cause' over his conduct of the investigation.
"Frankly it's unimaginable to me that Bob would ever do anything that gave rise to 'good cause,'" Barr told Senators.
Asked about a statement by one of the President's lawyers, Rudy Giuliani, that the White House should have the chance to 'correct' any public report by the Special Counsel, Barr frowned on that idea.
"That will not happen," Barr said.
"I believe the Russians interfered, or attempted to interfere, in the election and I think we have to get to the bottom of it," he added.
As Barr was testifying, Mueller's team filed new papers in a Washington federal court, asking for more time before sentencing Rick Gates, an aide and associate of former Trump campaign manager Paul Manafort.
In a court filing, the Special Counsel's office said Gates "continues to cooperate with respect to several ongoing investigations," as Mueller's team suggested another update in two months - indicating that the overall Russia probe is certainly not finished.