Passengers were able to travel on Metrolink's new service to south Manchester on Thursday.
The opening of the new service was delayed last month due to problems with aligning new computer systems.
However, a week after it was originally set to open, Transport for Greater Manchester announced the completion of comprehensive testing and commissioning work and the new line came into operation from the start of service at 6am on Thursday.
Cllr Andrew Fender, chairman of the Transport for Greater Manchester Committee, said: "Residents and businesses have been eagerly awaiting this news and I would like to thank them for their patience, understanding and continued support while we completed the essential testing and commissioning.
"I would also pass on my thanks and appreciation to all of our existing Metrolink passengers. Investment on this scale is bound to cause some disruption somewhere and I want to thank them for their patience too."
The new south Manchester line, with stops at St Werburgh's Road, Chorlton and Firswood, will give passengers a 15-minute link from Chorlton to St Peter's Square.
Services on the south Manchester line run:
- Every 12 minutes between 6am and 11.45pm, Monday-Thursday; 6am and 12.45am, Friday and Saturday; 9am and 5.30pm on Sundays
- Every 15 minutes between 7am to 9am and 5.30pm to 10.45pm on Sundays
TfGM said the new line forms part of a £1.4bn investment in Metrolink, which will provide new lines and new trams for a network that will be three times the size it is now.
Further lines are being delivered through Oldham and on to Rochdale, to Ashton-under-Lyne via Droylsden, from the Chorlton line to East Didsbury, and to Manchester Airport via Wythenshawe. A second line across the city centre is also being developed.