This coverage has been introduced within the tunnels from the Royal Oak portal, west of Paddington, to Liverpool Street station, covering around five miles.
This means users can now remain connected as they travel between stations.
Future coverage will focus on additional tunnelled sections of the Elizabeth line towards Whitechapel, where connectivity is expected within the coming weeks.
The entire line is projected to have mobile coverage by the end of summer.
London’s transport commissioner Andy Lord said: “It’s wonderful to see our programme to introduce high-speed mobile coverage now benefitting customers on the Elizabeth line, the newest part of London’s historic underground network of stations and tunnels.
“This key step in bringing better connectivity to London’s underground stations and tunnels will allow more people travelling around the capital to keep in touch, share photos and make the most of the city.”
Around 500 workers are working overnight to install mobile equipment across the Tube network, all of which is expected to be tidied away before customers start their morning commute.
Once finished, more than 2,000 kilometres of cabling along with thousands of radios are expected to be installed within tunnels and stations, all of which will be fitted outside of operational hours.
Boldyn Networks, a shared infrastructure provider, has won a 20-year concession by Transport for London (TfL) to deliver these improvements.
Billy D’Arcy, chief executive of UK and Ireland at Boldyn Networks, said: “Following the success of all underground Elizabeth line stations being connected, we’re delighted that TfL customers are now starting to enjoy 4G and 5G coverage in the tunnels.
“This will benefit a huge number of passengers travelling between Liverpool Street and Paddington, including tourists travelling through central London this summer, as progress continues on the rollout of our new digital infrastructure for London.”
Mobile connectivity is also extending to various Tube networks in central London, including the Northern line, Bakerloo line, Piccadilly line and Victoria line.
More than 30 Tube stations have started to offer coverage to customers in the ticket halls, platform areas and interchanges – with more expected to go live by the end of the summer.
As well as helping customers connect with their friends and family, this expansion also aims to provide better connectivity to rail and Tube staff so they can pass on information and host the new Emergency Services Network (ESN).
This network will enable first responders immediate access to life-saving data in live situations.
Michael Solomon Williams, head of campaigns at Campaign for Better Transport, said on the new Elizabeth line coverage: “This is great news for Elizabeth line passengers who will benefit from expanded mobile coverage.
“Extending coverage across London’s underground network will make it much easier for people to keep connected and help passengers to navigate their sustainable journeys by Tube.”
More information about the ongoing improvements in coverage can be found on the TfL website (tfl.gov.uk/get-online).