Georgia runoff winners Raphael Warnock and Jon Ossoff officially became senators, replacing the two Republicans they defeated. And California's Alex Padilla was sworn in after being appointed by Gov. Gavin Newsom to fill the vacated seat of Vice President Kamala Harris, who swore in the three.
All three represent historic firsts. Warnock is the first Black Democratic senator to represent a Southern state. Ossoff, 33, is the first millennial ever elected to the chamber. And Padilla is the first Latino senator from the diverse Golden State.
Their addition brought the Senate to a 50-50 split between the two parties, allowing Harris to recognize New York Democratic Sen. Chuck Schumer as the "majority leader" for the first time.
The move ends Sen. Mitch McConnell's reign as majority leader after six years. The Kentucky Republican, who won re-election to a seventh term in November, becomes minority leader.
Harris then swore in Sen. Patrick Leahy, D-Vt., as Senate president pro tempore, a job held by the longest-serving member of the chamber's controlling party and which is third in line to the presidency.
It was also a historic for Schumer, becoming the first Jewish Senate majority leader.
"That I should be the leader of the new Senate majority is an awesome responsibility," he said. "Today, I feel the full weight of that responsibility. A sense of reverence, of awe, of the trust placed in me."
Later in the day, the Senate confirmed President Joe Biden's first Cabinet member. Avril Haines was confirmed as Biden's Director of National Intelligence in an 84-10 vote.