"I'm glad to call you my colleagues," the Republican doctor from Athens told the chamber after getting a standing ovation from both sides of the aisle. "Just one week ago I was campaigning. Things have been moving very quickly since then, and I'm just overwhelmed."
Mr. Broun stunned Mr. Whitehead in the race to succeed the late Republican Rep. Charlie Norwood in Georgia's 10th District, which includes Athens and parts of Augusta.
Mr. Whitehead, a Republican endorsed by the party establishment, was the clear front-runner, with 44 percent of the vote to Mr. Broun's 21 percent in a 10-candidate special election June 19.
But in a July 17 runoff, Mr. Broun beat Mr. Whitehead by about 400 votes.
During Wednesday's ceremony, Mr. Broun was introduced by fellow Georgia Reps. John Lewis, D-Atlanta, and Jack Kingston, R-Savannah. Mr. Lewis noted that Mr. Broun's father was a popular state senator.
"I must say he was a Democrat," Mr. Lewis said, joking.
Mr. Kingston replied: "But he sure raised his son the right way."
Mr. Broun was sworn in by House Speaker Nancy Pelosi, D-Calif. He refrained from casting his first vote on an amendment to a spending bill that came up immediately afterward, saying he didn't know enough about the issue. His vote wouldn't have mattered because the amendment failed by several hundred votes.
Mr. Broun arrived in Washington on Tuesday night and began moving into Mr. Norwood's former office on Capitol Hill.
Still assembling his staff, he said he will stay through the week and begin participating in votes immediately. As a member of the Republican minority, he will serve on the Homeland Security and Science and Technology committees.
"The people of the district have not had a member for a number of months, and there's a backlog of issues that need attention," he said.
Without the benefit of group orientation sessions held after traditional elections, Mr. Broun has a lot to learn about navigating the Capitol.
At least temporarily, however, he will have one luxury usually reserved for more veteran members: choice office space. With seven terms under his belt, Mr. Norwood occupied prime real estate on the first floor of the Rayburn office building, a location that's far closer to the House floor than other newcomers' offices.
But the perk might not last for long. A spokesman for the House clerk said the office would be released into the general pool of available locations at the end of the session - where it will likely be snatched up by a more senior lawmaker.
According to results certified by the state Monday, Mr. Broun drew 23,529 votes in the runoff over Mr. Whitehead's 23,135 votes.
Mr. Whitehead conceded Tuesday after deciding against requesting a recount.
Gov. Sonny Perdue's office late Tuesday sent the official paperwork to the clerk of the House declaring Mr. Broun the winner, a spokeswoman said.
Mr. Norwood, a Republican, died after a long illness in February, at the start of his seventh term.