Paris, which has hosted two previous Olympics, will stage the event 100 years after its last Games in 1924 while Los Angeles will also organise its third Games after 1932 and 1984.
IOC President Thomas Bach embraced both cities' mayors on stage after a unanimous show of hands by IOC members ratified the awards.
The announcement was celebrated in Paris with the unveiling of a giant set of interlaced rings, the symbol of the Games, in a heavy downpour on the Trocadero square with lights glittering on the Eiffel Tower in the background.
"In the coming seven years we will prepare the Olympic Games with all our energy," French President Emmanuel Macron, who had personally campaigned for the bid, said.
"Together we must do everything in our power to ensure that Paris hosts a magnificent games in 2024."
The IOC decided in July to award both Games at the same time, following the withdrawal of four of the six cities bidding for the 2024 Olympics, amid concerns about the size, cost and complexity of organising the world's biggest multi-sports event.
Los Angeles then dropped its bid for the 2024 Olympics, for which it had been campaigning for over two years, in return for receiving the 2028 edition. This paved the way for Paris as the sole 2024 bidder to win the Games unopposed.
Momentous day
"This is a momentous day for the people of Los Angeles and the United States," said LA mayor Eric Garcetti. "For the first time in a generation, we are bringing the Games back to the City of Angels."
Paris, with a Games budget of 6.8 billion euros (6.12 billion pounds), had failed with previous attempts to land the 1992, 2008 and 2012 Olympics.
The LA Games have a budget of $5.3 billion and will essentially follow the plan they had in place for 2024, including housing athletes at the UCLA campus.
Both Paris's bid co-chair Tony Estanguet and LA bid chairman Casey Wasserman are to stay on and lead the host cities' preparations, mayors Anne Hidalgo and Garcetti confirmed in a news conference after the announcement.
It is the first time the IOC has awarded a Games 11 years in advance, as it traditionally gives the next Olympic host city seven years to prepare.
The Olympic body, however, was eager to secure the future of its prime product after Boston, Budapest, Rome and Hamburg all pulled the plug on their 2024 bids mid-race.
Bach said the unanimous IOC decision would secure the future of the Games and ensure successful Olympics.
"It is hard to imagine something better. Ensuring the stability of the Olympic Games for the athletes of the world for the next 11 years is something extraordinary," he said.
Both the 2014 winter Games in Russia's Sochi and the 2016 Rio de Janeiro Olympics were mired in controversy over corruption, embezzlement and doping allegations, dealing a major blow to the reputations of the event and the IOC.
Tokyo, the host of the next summer Games in 2020, has also had to fend off allegations of corruption regarding the decision to award it the Games four years ago.
