Ms Barrett said: "I will do my job without any fear or favour, and I will do so independently of both the political branches, and of my own preferences".
"Expand the court", tweeted the New York Democrat following the 52-48 vote.
The scene at an outdoor White House ceremony for Amy Coney Barrett looked very different from the previous White House event where President Donald Trump introduced her as his nominee.
A separate judicial oath, administered by Chief Justice John Roberts, is set to be administered tomorrow.
Amy Coney Barrett was sworn in as the newest justice of the U.S. Supreme Court by its longest-serving member, Justice Clarence Thomas, on Monday night, less than two hours after she was confirmed to the high court by the U.S. Senate.
"The rushed and unprecedented confirmation of Amy Coney Barrett as Associate Justice to the Supreme Court, in the middle of an ongoing election, should be a stark reminder to every American that your vote matters", Biden said in a statement.
Barrett's installation on the Supreme Court to serve what will potentially be a lifelong tenure will further consolidate the conservative majority on the bench, giving Republican appointees a 6-3 edge over those picked by Democratic presidents.
Democrats have argued that Republicans hypocritically backed Barrett's nomination just 38 days before the election, despite refusing to hold hearings for Barack Obama's nominee in 2016 after he nominated Merrick Garland to fill a vacant seat a full eight months before that year's vote.
No nominee to the Supreme Court has ever been confirmed by the Senate this close to a presidential election. Although Barrett's confirmation wasn't a surprise, groups like HRC, GLAAD, and the National Center for Lesbian Rights (NCLR) say having an ultra-conservative Justice like Barrett on the nation's highest court endangers the rights and freedoms of LGBTQ Americans. They have insisted that the victor of the election be allowed to decide who fills the vacant seat.
But Republican Majority Leader Mitch McConnell defended Barrett's nomination.
Get ready. abortion rights, the Affordable Care Act, LGBTQ rights, voting rights and tons of other fundamental issues and policies are now squarely in the crosshairs of the justices. Barrett's career has mostly been in teaching and she has only been a judge for all of three years. Democrats accused Republicans of rushing Barrett through her confirmation process so she could rule against the ACA, as she had previously criticized the 2012 decision that upheld it.