Kirk takes over the seat occupied by Democrat Roland Burris and once held by President Obama.
Kirk will begin serving out the remainder of Obama's elected term in the U.S. Senate.
Kirk will be a Republican voice in the lame duck session, but will have to endure some short-term inconveniences
His supporters never dreamed that swearing-in day for U.S. Senator-elect Mark Kirk would begin in a cramped space in three makeshift rooms in the courtyard of the Russell Senate Office Building next to the U.S. Capitol.
"Right now, I'm in a double wide," Kirk said Monday.
In a few days, Kirk will move into the office vacated by the senator he replaced, Roland Burris. Then, next year, Kirk will be part of a lottery for new senators and likely will move again.
It's the price he must pay for a few weeks seniority over other new senators and the opportunity to vote during the lame duck session of Congress.
"I think the mandate from the whole country was: have the Congress work on the economy first, second and third, to build up a new employment base in the United States," Kirk said.
Kirk says his first order of business will be to vote against any new spending programs in a federal budget he and other new Republican senators have vowed to cut.
"This budget reduction in my view needs to be across the board. We need to be sure that we don't raise $2 trillion in tax revenue but then spend $3 trillion," said Kirk.
The swearing-in ceremony was attended by Kirk's mother and sister.
The self-described moderate says he'll vote against the Dream Act that allows the children of undocumented immigrants to attend U.S. colleges and join the armed services, and he says he's undecided on the START Treaty and the military's "don't ask, don't tell" policy for gays.
Kirk also says he will not join other senators who have said publicly they are working to make Barack Obama a one-term president.
"I am not anti-Obama," Kirk said. "I'm pro American, pro Illinois. There is room for bipartisan cooperation on the budget."
Kirk was also accompanied at the swearing-in by Democrat Dick Durbin and Peter Fitzgerald, who held Kirk's seat between 1999 and 2005.
"Senator Kirk really needs to do what he thinks is right. Can't worry about the politics," said Fitzgerald.
"I think he'll be an aide in helping to bring different factions together," said Rep. Danny Davis (D).