Barrett took the constitutional oath of office before President Donald Trump at a brief ceremony hosted by the White House on Monday night.
Coming through a 53-48 vote along party lines, Barrett’s confirmation is seen as a victory for Trump, who nominated her following the death of liberal justice, Ruth Bader Ginsburg.
Shortly after the ceremony, Speaker of the House of Representatives, Ms Nancy Pelosi, issued a statement accusing Trump and Senate Republicans of committing “an act of supreme desperation”.
Pelosi said they rushed through a Supreme Court nominee just eight days before the Nov. 3 presidential election “as part of their years-long campaign to destroy Americans’ health care”.
Democrats are worried that the appointment of Barrett, a conservative, will favour Republicans in politically sensitive cases pending before the apex court.
One of the cases is the Affordable Care Act enacted under the administration of former President Barack Obama which Trump has been trying to scrap.
Democrats argue that no fewer than 20 million Americans could lose their health care coverage if the apex court repeals the law, otherwise known as Obamacare.
“In her confirmation hearing, Justice Barrett refused to answer whether she believes that Medicare is constitutional.
“She refused to acknowledge the scientific fact that climate change is happening, saying she isn’t a scientist, calling it ‘controversial,’” the speaker said.
Democratic Party’s Vice Presidential candidate, Ms Kamala Harris, described Barrett’s confirmation by the Republican senators as despicable, saying they must be held accountable.
“Instead of working to provide COVID-19 relief to struggling Americans, Mitch McConnell and Republicans chose to jam through a Supreme Court nominee — when more than 62 million people have already voted.
“It’s despicable and they must be held accountable,” Harris said on Twitter.
Former First Lady and Secretary of State, Mrs Hillary Clinton, also tweeted her reaction following the confirmation.
“Republicans just pushed through a Supreme Court justice who will help them take away Americans’ health care in the middle of a pandemic,” she said.
However, Barrett said in her speech after she took the first oath of office that her policy preferences would not influence her decisions on cases.
She said the confirmation process had made one of the fundamental differences between the federal judiciary and the Senate clearer to her.
“And perhaps the most acute is the role of policy preferences. It is the job of a senator to pursue her policy preferences.
“In fact, it would be a dereliction of duty for her to put policy goals aside. By contrast, it is the job of a judge to resist her policy preferences.
“It would be a dereliction of duty for her to give into them. Federal judges don’t stand for election.
“Thus, they have no basis for claiming that their preferences reflect those of the people.
“This separation of duty from political preference is what makes the judiciary distinct among the three branches of government,” she said.
Speaking earlier, Trump said Barrett’s confirmation was a “momentous day for America, the United States Constitution, and for the fair and impartial rule of law”.
“The Constitution is the ultimate defense of American liberty, the faithful application of the law is the cornerstone of our republic.
“That is why as president I have no more solemn obligation of no greater honor than to appoint Supreme Court justices”, Trump said.
Barrett’s lifetime appointment has sealed a 6-3 conservative majority in the nine-member Supreme Court bench.
All Democrats in the Senate voted against her confirmation, which came just a week before the presidential election.
Republican senator, Susan Collins, representing the northeastern state of Maine, joined the Democrats in opposing the confirmation.(NAN)