But the works, which included installing new sewers as well as gas and water pipes, have now been completed and the road is due to open again in both directions by 6am on Saturday, July 28 ahead of the start of the Edinburgh International Festival and the Fringe.
However, one lane northbound will remain closed for safety reasons until the development is completed in 2020.
And work is still continuing on reconfiguring the Picardy Place roundabout despite hopes it could be ready in time for the return of traffic to Leith Street.
The re-opened street features reconstructed, widened pavements, four primary crossing points and traffic signals at the junction with Calton Road.
There will be a dedicated two-way cycle lane between Calton Road and Picardy Place.
Carol Nimmo, chair of New Town and Broughton Community Council, welcomed the prospect of Leith Street re-opening. “We can’t wait,” she said. “We were worried it wasn’t going to re-open to cars and vans and it would just be for public transport.
“We need these routes linking the Old and New Towns and Leith Street is a vital junction.”
She said the closure had led to congestion in other parts of the city centre.
“It has skewed traffic hugely – and traffic had to wind its way through the New Town to get to other routes,” she said.
Buses will start using Leith Street again from July 28 at 5am.
Services returning to Leith Street are 1, 4, 5, 7, 8, 14, 19, 22, 25, 34, 45, 49, N7, N11, N16, N22, N25, N26, N34 and N44. A new service 35 between Ocean Terminal and Riccarton Campus will also use the route.
The developers said the work on Leith Street had been completed on schedule and there would be no need to close the road again after the festival.
Martin Perry, director of development for Edinburgh St James, said: “We would like to thank the public for their patience during the main phase of construction work along Leith Street and we are delighted to now be re-opening the street to traffic as planned.”
Transport and environment convener Councillor Lesley Macinnes also hailed the return of traffic to the street.
She said: “The re-opening of Leith Street marks the culmination of the first phase of public realm improvements being delivered in conjunction with the Edinburgh St James project.
“With wider pavements and better cycling provision, the works have created a much more accessible and people-friendly thoroughfare.
“Temporary traffic management will remain in place to allow construction of the new Picardy Place road layout to continue. I would like to thank the public for their continued patience and co-operation while these works are carried out.”
Cllr Macinnes said a public conversation would continue after the summer on what should go in the central island at Picardy Place and on details of the area outside the RC cathedral.