Almost £10m was spent on improvements inside and outside of the building.
Work began in 2010 and changes include a new visitor centre, disabled access and a patio cafe.
The facade has been given a facelift, and the roof and clock have been repaired. The building is due to open to the public on 10 June.
Mayor Kevin Campbell said the completion marked a fantastic day for tourism and for the city.
"I think the public, when they get in on 10 June, will think it's absolutely fantastic," he said.
"It still holds its mystique and its character going back over 150 years but at the same time it has been modernised.
"Between the walls and the peace bridge the Guildhall is the last piece in the puzzle for the city."
Other changes include the mayor's office becoming a tourist point for visitors to get information on where to go in the city.
The room previously used as the council chamber will now host a "touch screen" museum telling the story of the city, including its 400-year-old walls and the plantation.
A major refurbishment of Londonderry's Guildhall has been completed and the building is due to be officially handed over to the council on Thursday.