The decision comes after a delay because of a number of issues that kept the network from rolling out the faster internet service. It had been supposed to arrive i 2019, but had to be pushed back.
It means that it is the last of the major networks to introduce the new wireless internet technology for its customers.
Three will give people access to the 5G network for free, so long as they have a phone capable of accessing it.
The telecommunications company has now revealed that its 5G offering will be available on mobiles in 65 locations from the end of February, including London, Cardiff, Glasgow, Manchester, Birmingham, Coventry and Nottingham.
The company first made its 5G available to homes in August, as a broadband service across parts of London.
To mark the expansion to smartphones, Three has partnered with the Central Saint Martins art school to create an immersive 5G experience for London Fashion Week, using spatial audio, haptic feedback, a 46 metre projection the length of the runway, and an array of aromas.
The finale will culminate in an empty catwalk, with a 5G-fuelled digital version of model Adwoa Aboah, who will walk out in the last outfit of the show.
"Today we are celebrating what is possible through 5G with a showcase of our ultra-fast 5G capabilities marking the next step in our 5G journey," Three chief executive Dave Dyson said.
"5G is set to change the world for all of us and we can't wait for our customers to start experiencing it."
Rivals EE launched their 5G offering to smartphones in May, followed by Vodafone in July and O2 in October.
Additional reporting by agencies