The ordinance, signed Tuesday by Mayor Antonio Villaraigosa, takes effect Oct. 1 and is aimed at reducing and eventually eliminating the thousands of animals that are euthanized in the city's animal shelters every year.
"We will, sooner rather than later, become a no-kill city and this is the greatest step in that direction," Councilman Tony Cardenas, who co-authored the bill, said as he held a kitten at a City Hall news conference.
Another of the ordinance's co-authors, Councilman Richard Alarcon, brought his two pet Chihuahuas to the event to be neutered in a van operated by the city.
The ordinance brings the nation's second-largest city in line with about a dozen of its neighbors that have similar laws.
It does exempt some animals, including those that have competed in shows or sporting competitions, guide dogs, animals used by police agencies and those belonging to professional breeders.
The average pet owner, however, must have their dog or cat spayed or neutered by the time it reaches 4 months of age -- as late as 6 months with a letter from a veterinarian.
First-time offenders will receive information on subsidized sterilization services and be given an additional 60 days. If they still fail to comply, they could be fined $100 and ordered to serve eight hours of community service.
People with older unneutered pets and newcomers to the city with animals also have to obey the law.
Los Angeles animal shelters took in 50,000 cats and dogs last year and euthanized approximately 15,000 at a cost of $2 million, according to city officials.
Longtime animal-sterilization advocate Bob Barker of "Price is Right" fame pushed for the law's adoption and was among those at Tuesday's news conference.
"The next time that you hear me say, 'Help control the pet population, have your pet spayed or neutered,' I can add, 'It's the law in Los Angeles,'" Barker said.