The footage was published by Louisiana-based newspaper The Quachita Citizen and clearly showed the married man and father of five participating in an extramarital kiss.
The woman he was captured on video making out with for 30 seconds is his district scheduler, 33-year-old Melissa Anne Hixon Peacock. She is also married and joined McAllister's congressional staff after he took office in November, the newspaper said, citing the Federal Election Commission.
The news outlet says she donated $5,200 to his congressional campaign last fall, and listed her employment at the time of the donation as a self-employed cosmetologist.
McAllister, who ran on a platform of family values, asked for forgiveness Monday night after the newspaper posted the video on its website.
"There's no doubt I've fallen short and I'm asking for forgiveness. I'm asking for forgiveness from God, my wife, my kids, my staff and my constituents who elected me to serve," McAllister said in his apology.
The Republican won a seat in the House through a special election last November after his predecessor Rep. Rodney Alexander resigned.
He campaigned as a Christian conservative, "always tried to be an honest man" and ran "to make a difference and not to just be another politician."
The 40-year-old benefited from support from the "Duck Dynasty" fame. Phil Robertson publicly supported McAllister, and Willie Robertson endorsed McAllister in a YouTube video. In January, he also made headlines when he invited Willie to attend President Barack Obama's State of the Union address as his guest.
The Quachita Citizen says the controversial video was shot inside McAllister's congressional office in West Monroe on Dec. 23, 2013. How the news outlet obtained the video remains unknown.
"Trust is something I know has to be earned whether you're a husband, a father, or a congressman," he continued. "I promise to do everything I can to earn back the trust of everyone I've disappointed."
McAllister faces a re-election campaign this year.
ABC News and The Associated Press contributed to this report.