McAllister, who largely self-funded his campaign, beat establishment candidate Neil Riser, a state senator, in a special runoff election for the vacant 5th District seat.
Both men are Republicans.
With almost all of the vote counted, McAllister had a commanding lead.
McAllister, a businessman with multiple companies, ran as a political outsider, capitalizing on frustration with politicians and Congress. As a point of pride during the campaign, he said he'd never been to Washington.
Riser, a funeral home owner in the Senate since 2008, campaigned on his experience in the Legislature and with the support of tea party groups.
"Plain and simple, this was Riser's election to lose. Riser was the favorite going into the evening. He had the dollars. He had the endorsement of the Republican establishment. He had a strong showing in the primary. Yet, he lost it," said Joshua Stockley, a political science professor at the University of Louisiana at Monroe.
Riser and McAllister are both conservatives and largely agreed on many issues. Both oppose abortion, favor strong gun rights and criticize the levels of federal spending and debt.
Their sharpest distinction rested with President Barack Obama's signature health care law.
Both opposed the health overhaul, but Riser wanted only repeal, saying the law will harm businesses and families and can't be fixed.
McAllister said repeal had no chance with Democrats leading the Senate and White House, so he said Congress should work to improve the law. He also wants Louisiana to expand its Medicaid program to give insurance to the working poor, an expansion that Riser opposes.
The positions put McAllister at odds with some tea party supporters but generated support from Democrats who had no candidate of their own in the runoff.
Stockley said voters in the very strongly conservative 5th District "signaled that McAllister's pragmatism seems to be a more tenable governing solution."
McAllister will represent a largely rural district along the Mississippi River delta dotted with farmland and plagued by poverty. The 5th District covers all or part of 24 parishes, from northeast and central Louisiana into southeastern parishes bordering Mississippi.
The congressional seat was vacant because Republican Rodney Alexander decided to leave Washington in the middle of his sixth term and take a job in Gov. Bobby Jindal's administration. In the race to succeed him, Alexander was supporting Riser, an ally of Jindal.
Best known for a constitutional provision toughening Louisiana gun rights, Riser announced his campaign immediately after Alexander announced his departure, raising accusations that Jindal and Alexander tried to influence the election for Riser. All three denied the claim.
"Riser got punished for being too closely allied to Gov. Jindal," Stockley said.
McAllister's ability to reach the runoff in a pack of 14 candidates, including six elected officials, caught many by surprise. He made the runoff with little outside help, no prior name recognition and no heavyweight fundraising.
His friendship with "Duck Dynasty" patriarch Phil Robertson helped draw attention to his long-shot bid for office, and McAllister's own deep pockets paid for most of his campaign expenses.
McAllister spent at least $800,000 of his own money on the race, according to the Federal Election Commission. By comparison, Riser raised a similar amount and put no personal wealth into the campaign.