Retired Justice Anthony Kennedy, whom Mr Kavanaugh once clerked for and whose place he has taken on the court, administered the oath of office.
The tone inside the courtroom was collegial, in contrast to the venom of Kavanaugh's confirmation fight.
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". Speaking in the favour of Kavanaugh, Trump said, "You, sir, under historic scrutiny, were proven innocent".
Senators had voted 50-48 on Saturday to confirm Mr Kavanaugh's appointment after three women made allegations of sexual misconduct, which he emphatically denied.
President Trump told reporters at the White House on Monday that the American public wouldn't put up with an attempt to unseat Kavanaugh.
Questions from Kavanaugh and Justice Neil Gorsuch, President Donald Trump's two high-court picks, suggested they could vote against the Trump administration and side with a criminal defendant from Florida who is fighting an increase in his sentence from just over six years to possibly more than 15 years.
Kavanaugh seemed aware of the cloud that hung over his ascension to the nation's highest court and attempted to downplay the partisan tensions that burned during the Senate's consideration of his nomination. Fred Schilling/Supreme Court of the United States. Kamala Harris turned up in politically important Ohio, where she reminded more than 1,000 of the party faithful at the Ohio Democratic Party's fall fundraising dinner that she walked out of the Kavanaugh proceedings at one point because they had become "a sham and a disgrace".
"Watching last night's White House event and listening to the president again cast doubt on the veracity of Dr. Ford's claims, while not hearing a word of contrition from the newest justice, was hard for many Americans who have experienced sexual misconduct firsthand", Roth added.
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While liberal activists have called for impeaching Kavanaugh, many Democratic lawmakers have tried to tamp down such talk in recent days.
After an extra Federal Bureau of Investigation probe - which media reports say was drastically curtailed by the White House - also found nothing new, Kavanaugh was finally voted into the coveted post. But percolating in the courts below are major issues - concerning the Affordable Care Act, DACA and LGBT rights that could be headed to the Supreme Court perhaps as early as this term.
Nearly immediately after the Senate vote, Democrats felt the chill from faraway North Dakota.
Both Kavanaugh and Blasey Ford spoke before the Senate Judiciary Committee in an extraordinary day of raw testimony.
Only one Democrat, Joe Manchin of West Virginia, voted for Trump's nominee.
Kavanaugh moved to the Supreme Court after spending 12 years as a judge on the influential U.S. Court of Appeals in Washington, building a conservative judicial record.
Conservative Justice Neil Gorsuch joined the court previous year. "President, you will not fill the Supreme Court vacancy, '" McConnell famously said in August 2016.
Republicans hold a 51-49 majority in the Senate, with several seats up for grabs in November.