Chief Justice John Roberts administered the oath to Kagan in a brief private ceremony at the court. Kagan, joined by family and friends, pledged to faithfully and impartially uphold the law.
Afterward, she smiled broadly as a crowd of onlookers stood and applauded. "We look forward to serving with you," Roberts said.
Kagan, a former Harvard Law School dean who most recently was solicitor general, was President Obama's choice to succeed retired Justice John Paul Stevens. Republicans criticized her as a political liberal, before the Senate confirmed her this past week on a vote of 63-37.
She was sworn in twice Saturday by Roberts — reciting one oath as prescribed by the Constitution during a ceremony in a conference room at the court with only her family present. Kagan then recited a second oath, taken by judges, with her family and friends and reporters present.