Transport bosses have always highlighted 2020 as the opening of the route along Pipers Row, but Andy Street says services probably won't be up and running until the end of that year.
He said that while the Metro section is "ready to go", workers would have to wait until the completion of the new railway station terminus so the two can be linked.
The terminus is part of a £150m project to link the two modes of transport.
The Mayor said: "The tracks are already laid and the Metro part of that is ready to go.
"The issue is that the terminus is right outside what will be the new Wolverhampton [railway] station. We have to wait to link in with that.
"So that will probably be the end of 2020. It's very much dependant on the station works, not the Metro works."
Mr Street was speaking as the first tracks were laid in the latest phase of the region wide Metro extension, which bosses claim will fully connect the West Midlands and bring jobs and prosperity.
Trams are on course to run between Grand Central and Centenary Square in Birmingham by the end of this year, while with the Broad Street to Edgbaston route set to be up and running by late 2021.
The Wednesbury to Brierley Hill extension is planned to start running in 2023.
Councillor Roger Lawrence, the leader of Wolverhampton Council and transport boss with the West Midlands Combined Authority, said: "We are trebling the size of the tram network in the next few years and this is to make sure we best link to HS2 and get the maximum economic benefits from the high speed rail line.
"These routes, particularly in the Black Country, will unlock new housing and employment sites and give more people access to jobs."
Mr Street said the £70m extension in Birmingham was "a clear sign" that he was delivering "a revolution in public transport" across the region.