Kirkpatrick will return to the seat she represented from 2008 to 2010, when she was defeated by Rep. Paul Gosar (R).
The race was one of 10 that remained undecided as late as Wednesday afternoon, but with 99.4 percent of precincts reporting, Kirkpatrick took 48.5 percent of the vote to Paton's 45.3 percent, a difference of more than 7,000 votes.
The 1st Congressional District was a top target for both Democrats and Republicans, and outside groups spent over $6 million on the race.
The district was more Democratic than Republican, but Arizona's red lean gave Republicans hope that they could pull out a win.
Republican attacks on Kirkpatrick centered around her support for President Obama's policies during her first term in Congress, while Democrats targeted Paton for his work as a lobbyist for the payday loan industry.