During a program, Verizon launched its 5G services in Chicago and Minneapolis on Wednesday itself.
Experts say 5G will bring smartphones near-instantaneous connectivity - 20 times faster than 4G - allowing users to download entire movies in less than a second.
The new 5G technology enables dramatically faster download speeds with nearly no delay in watching videos or loading pictures.
By beating the United States and China to the punch, it provides a major selling point for Samsung Electronics Co. against Apple Inc.in premium smartphones and Huawei Technologies Co.in the market for mobile network gear.
However, speculation that U.S. mobile carrier Verizon might start its 5G services early forced South Korean providers to hastily organise a late-night launch, the Yonhap news agency reported.
Park Jung-ho, CEO of SK Telecom, talks up the operator's 5G offering on Wednesday at what SKT called the "beginning of the Age of Hyper-Innovation with 5G" at the company's 5G Launching Showcase.
Samsung's Galaxy S10 5G goes on sale in South Korea
SK Telecom spokeswoman Irene Kim told Reuters the company had internal discussions and chose to launch the 5G service early as the company had networks and customers ready.
South Korea is all over the news media outlets from the past couple of days and now in yet another remarkable feat, the nation has witnessed the launch of world's first 5G phone.
For general customers, the services will be available from Friday - the original launch date - when Samsung Electronics rolls out the Galaxy S10 5G, the world's first available smartphone with the technology built in. Rival KT Corp is set to offer cheaper plans than its LTE service, with unlimited data and 4-year instalments to buy 5G devices. Ordinary consumers in South Korea will be able to connect to 5G services starting tomorrow, when Samsung's Galaxy S10 5G, the first smartphone with built-in 5G, goes on sale.
LG Electronics plans to release its 5G smartphone in South Korea later this month.
Chinese firms have invested heavily in the technology, while Japan's mobile carriers plan to have a full 5G service in place in time for the 2020 Tokyo Olympics.
The launch was reportedly brought forward by two days, as speculation mounted that USA mobile carrier Verizon planned to launch its own service ahead of schedule and claim the title of the world's first 5G provider. The country has one of the world's top smartphone penetration rates.
Golden State Warriors trio fined for criticising officials
The next tie-breaker is conference record, and Denver (31-15) now holds a slight edge there over Golden State (31-16). The big man stole the ball from Nikola Jokic and took it coast-to-coast for a left-handed layup. "Pretty cool moment.
Brexit: Downing Street 'ready to meet Labour this weekend'
The UK Chancellor has said his government has "no red lines" in their talks aiming to find a Brexit compromise with Labour. Sterling fell against the USA dollar immediately after Labour's comments and stood 0.5 percent down on the day.
President's comment about wind turbines causing cancer confuses Iowa's Chuck Grassley
She declined to say whether Trump was wrong about wind turbines' connection to illnesses. Kamala Harris, D-Calif., Kirsten Gillibrand, D-N.Y., and Amy Klobuchar, D-Minn.