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.

The Crown Prosecution Service (CPS) used a confiscation order to seize John Dawes' assets after he was convicted of drug dealing and money laundering.
Helen Dawes maintained she was innocent of any wrong-doing and that her role was simply to run the household.
But the judge disagreed and said she was "shutting her eyes" to the obvious.
The CPS launched the High Court action to seize Dawes' assets after he was jailed in 2005 for 24 years.
But his wife fought the confiscation order claiming she was not party to her husband's activities and that she had a 50% stake in the home, valued at £177,486.
She claimed her role was simply to look after the house, their son and do the cooking while Dawes ran the finances.
But on Tuesday, Mr Justice Collins said: "Mrs Dawes turned a blind eye to what was going on, not because she necessarily did not want to know, but because she took the view that her part was merely to run the home, look after her child and provide for her husband."
He added: "It seems to me she has been shutting her eyes to what should have been obvious."
Dawes was estimated to have earned £8,283,161 between 1997 and 2003 from drug crime and he was ordered in September 2008 to pay back £355,000.
The court heard Mrs Dawes and her husband had several bank accounts with significant sums of cash passing through.
The amount of money involved was far in excess of what the couple legitimately earned.
Mrs Dawes has since moved in with her elderly father.
Mr Justice Collins said there was no reason why their son, who is in full time education, could not find somewhere else to live now he is over 18.
Dawes lost an appeal against his conviction in 2007 but maintains his innocence.

The 18-year-old man was taken to hospital with multiple stab wounds following the altercation in Bowleymead in the Eldene area on Saturday.
Daniel Peapell, 20, and Zac Doyle, 19, both from Swindon, were charged alongside 19-year-old Raffaele Bretti from Wroughton.
The three were remanded in custody by Swindon magistrates.
They will all next appear at Bristol Crown Court in July.
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.
[NEXT_CONCEPT]
The wife of a jailed Nottinghamshire drugs baron will lose the family home after a High Court judge ruled she turned a blind eye to his activities.
[NEXT_CONCEPT]
Three men have been charged with the attempted murder of a teenager who was stabbed repeatedly in Swindon.