"For me this has been an amazing personal journey," said the former first lady, who was the early favorite for the Democratic presidential nomination last year until Obama beat her in a grueling campaign.
Biden also ran for president last year, but bowed out after early losses to Clinton and Obama.
"As Joe laughingly referenced, neither one of us thought we would be standing here together doing what we are now doing. Life has that funny way of unfolding and politics is even stranger," she said.
Clinton placed her hand on a Bible held by her husband, the former president, as she repeated the oath administered by Biden. She was sworn in privately on Jan. 21 after she was confirmed by the Senate.
It'll be interesting to see how this plays out. Can they really put the bitterness of the primary campaign aside? And will people constantly refer to Clinton's husband, despite the fact that she's the one in power now?