"On behalf of the nation, I'd like to apologize to Brett and the entire Kavanaugh family for the awful pain and suffering you've been forced to endure", President Trump said.
In court, Kavanaugh asked questions of both sides in arguments over increased prison sentences for repeat offenders.
Fulfilling a promise from his confirmation hearing, Kavanaugh has hired four female clerks - something that no other Supreme Court justice has ever done. "Every litigant at the Supreme Court can be assured that I will listen to their argument with respect and an open mind", the newly minted Justice added.
Kavanaugh's Saturday confirmation in one of the closest such Senate votes in history highlighted deep American polarization ahead of November 6 congressional midterm elections where Democrats hope to end Republican dominance.
Kavanaugh officially took the oath in a more hurried, private procedure Saturday, but the White House version late Monday was a chance for the Trump administration to perform the equivalent of a victory lap.
The president - whose Republicans fear losing at least the lower chamber of Congress - predicted that Democrats would pay for their attempts to block the confirmation, especially during the lurid debate over decades-old sexual assault allegations.
In front of a packed courtroom, Chief Justice John Roberts welcomed the newest judge on the nine-member panel and wished him a "long and happy career".
Kavanaugh's nomination had appeared safe until Christine Blasey Ford, a university professor in California, last month went public with allegations that Kavanaugh had sexually assaulted her in 1982 when they were high school students in Maryland.
The cases challenge the types of crimes that qualify as violent felonies under that law and can lead to 15-year mandatory minimum sentences for a defendant.
Kavanaugh's ceremonial swearing-in has some historical precedent, as most of the current members of the Supreme Court had some White House aspect to their oath-takings, even if they already had been done so privately and even started work.
Kavanaugh was Trump's second selection for the nation's highest judicial body, following conservative Justice Neil Gorsuch previous year.
Mr Kavanaugh denied the allegations and during a Senate hearing on September 27 accused Democrats of an "orchestrated political hit".
Supreme Court Justice Brett Kavanaugh has been sworn in -again - at an event at the White House, but not before President Donald Trump slammed Kavanaugh's opponents for a "campaign of personal destruction".
"I'm not saying nobody could do it. but it requires a lot of force, more than you might think", Roberts said. Kavanaugh replaced Kennedy on the bench.
The groups - Be A Hero Team, Maine People's Alliance and Mainers for Accountable Leadership - said Collins' re-election campaign would have received the money if she'd voted against Kavanaugh's confirmation. That's because the Kavanaugh fight undoubtedly galvanized both parties' bases, and in places with key Senate races like Tennessee and North Dakota that's certainly not helpful, as anything that polarizes both sides equally in the national fight will only help Republicans in Republican strongholds.