As the longest serving Democrat now in the Senate, Leahy moved to third place in the line of presidential succession, behind Vice President Joe Biden and House Speaker John Boehner.
Biden swore Leahy in as the successor to Hawaii Sen. Daniel Inouye in the Senate post. Inouye, who also chaired the Senate Appropriations Committee, died Monday at age 88. A vase of white roses was on Inouye's desk on the Senate floor.
"I can't tell you how much it pains me," Leahy said before being sworn in. "He was one of the greatest members of this body ever to have served, and a dear friend to so many of us."
Leahy is expected to succeed Inouye as chairman of the appropriations panel. The 72-year-old Leahy is the current chairman of the Senate Judiciary Committee.
Sen. Dianne Feinstein of California is expected to take over from Leahy as chairman of the Judiciary panel, a move that could boost the prospects for congressional action on gun control measures in the wake of the Newtown, Conn. school slayings. Feinstein has promised to reintroduce the expired federal ban on assault rifles.
Sen. Barbara Mikulski, D-Md., is expected to become chairman of the Select Committee on Intelligence, replacing Feinstein on that panel.
An announcement on the new chairmanships could come as soon as Wednesday.
Leahy was elected in 1974 and is serving his seventh term. He was the youngest senator to be elected in Vermont, at age 34, and is the state's longest serving senator.
A former prosecutor, Leahy has been active on criminal justice, human rights, privacy and environmental issues during his Senate career.
After the Sept. 11, 2001 terrorist attacks, Leahy led the Senate's negotiations with the Bush administration on the USA Patriot Act, the sweeping anti-terrorism bill responding to the attacks. He was a strong advocate in the international campaign against the production, export and use of anti-personnel land mines, writing the first law by any government to ban the export of the mines.
Leahy was a leader in the Judiciary Committee's investigation into the mass firings of U.S. attorneys during the Bush administration.
He is also an amateur photographer. A longtime "Batman" fan, Leahy and has made cameo appearances in "Batman" films.
Leahy and his wife Marcelle Pomerleau Leahy have a daughter and two sons. They live on a tree farm in Middlesex, Vt.