Councilman Eric Garcetti won 33% of the vote, while Controller Wendy Greuel took 29%, preliminary results showed.
BBC: Run-off election for Los Angeles mayor after split poll
Meanwhile, Ms Greuel pledged to "work on the tough issues like education and jobs and getting LA back on track".
BBC: Run-off election for Los Angeles mayor after split poll
Greuel's attack ads hit Garcetti for a fundraiser organized by a developer who she says once served prison time for fraud.
"No one said it was going to be easy or quick, " Greuel told a crowd of supporters as votes were being counted.
Greuel got the other 46%, figures from the City Clerk's Office show.
"I voted for Wendy Greuel, " said Chris Johnson, 31, a college employee.
Greuel and Garcetti emerged from a March primary in which no candidate secured the majority needed to win outright, leading to Tuesday's runoff.
If elected, Mr Garcetti would become Los Angeles' first Jewish mayor, while Ms Greuel would be the first woman to hold the office.
BBC: Run-off election for Los Angeles mayor after split poll
Wendy Greuel was hoping to become the city's first female mayor.
NPR: With White House Bogged Down By Scandal, GOP Looks For Boost
The preliminary numbers pegged Garcetti at 52 percent and Greuel at 48 percent, with about 6, 100 votes separating them out of about 176, 000 counted so far.
Garcetti's commercials labeled Greuel "DWP's mayor, " a reference to the Department of Water and Power, whose workers financed ads to help install her at City Hall.
Greuel, 51, would have been Los Angeles' first female mayor.
His opponent, Wendy Greuel, did not immediately issue a statement.
First of all, a lot of money was spent mostly on - by Greuel and her labor allies, and apparently Garcetti hit that, hit the fact that Greuel was supported by labor unions to his advantage.
NPR: With White House Bogged Down By Scandal, GOP Looks For Boost
He defeated the city controller, Wendy Greuel.
NPR: With White House Bogged Down By Scandal, GOP Looks For Boost
On Monday, Greuel sent off a final round of recorded endorsements from former President Bill Clinton, in whose administration she once worked, while Garcetti was on campaign stops arguing a simple point to mostly indifferent residents: The election matters.
Eric Garcetti, 42, and Wendy Greuel, 51 - both veterans of city hall - have been cited as mayoral candidates who might face each other in a run-off vote in May if no candidate wins a majority of the vote in Tuesday's election.
BBC: Los Angeles mayoral election under way amid low turnout
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