JOHANNESBURG >> The central Johannesburg building where Nelson Mandela and Oliver Tambo had a historic law office was on the verge of collapse. Now, city officials hope it will anchor a renaissance.

Amos Masondo, the mayor of South Africa’s largest city, on Wednesday showed off the results of 5 million rand (about $750,000) spent over the past year to turn Chancellor House into a museum and archive. Mandela and Tambo, who each would go on to lead the African National Congress, opened the country’s first black law partnership on the top floor in 1952 and closed it in 1960, when their political work made it impossible to keep practicing.

Masondo has another 2 million rand (about $300,000) budgeted for the finishing touches of the renovation and to install the planned museum tracing the building’s history and the digital archive of cases Mandela and Tambo handled. He said he hopes private businesspeople will now be drawn to the area and help drive broader renewal for a dilapidated neighborhood. He said he wants to “leverage heritage to ensure development.”

Mandela and Tambo held political meetings at Chancellor House, and it was a hub of legal preparations for those arrested during the 1952 Defiance Campaign, when blacks were encouraged to break racial separation laws, and during the 1956 treason trial, when Mandela and Tambo were among 156 defendants charged for supporting the Freedom Charter calling for a nonracial democracy.

Both leaders represented blacks who had run afoul of apartheid-era laws by committing acts such as riding on white-only buses or drinking from fountains reserved for whites.

In “Long Walk to Freedom,” Mandela writes that for black South Africans, the Chancellor House offices were “a place where they could come and find a sympathetic ear and a competent ally, a place where they would not be either turned away or cheated, a place where they might actually feel proud to be represented by men of their own skin color.”

Mandela’s own lawyer, George Bizos, said Wednesday that tradition must continue. Bizos is working with other lawyers to raise funds to open a law library at Chancellor House and maintain offices there for lawyers who cannot afford offices in the area.

The goal is “to make it not a monument, but a living structure, a living place in honor of particularly Nelson Mandela and Oliver Tambo,” Bizos said.

Chancellor House is just across the street from the city’s stately main magistrate’s court. But telltale signs — broken windows, rusted roofs — attest that other neighboring buildings have been abandoned. As apartheid ended, white business and homeowners in downtown Johannesburg fled, or simply stopped maintaining their property.

Renewal efforts have had some successes in recent years, but it sometimes seemed Chancellor House was to be forgotten, despite the efforts of historians and activists. Squatters had moved in, and Mayor Masondo said the building’s owners had considered tearing it down and putting up a parking garage. In 1999, efforts began to have it declared a national monument. The city negotiated to buy the building, but Masondo said the effort stalled and in the end the city took it over.

The takeover was completed, allowing renovations to begin in June 2010. Nkosinathi Manzana, chief operating officer of the city’s development agency, said 30 tons of debris had to be removed just so experts could get in and determine whether the building could be saved.

Water damage and fires had seriously weakened the building. In the end, renovators had to erect a steel structure to support the building and replace its roof. From the outside, Chancellor House looks much as it did when it was built in the early 1940s.

Manzana said wood-and-glass front doors date from Mandela’s day, but little else was salvageable after decades of neglect and vandalism. Architects searched antique shops to find parquet flooring for the ground floor and the top floor offices where Mandela and Tambo worked.

Mandela, 92, became South Africa’s first black president after serving 27 years in prison for his fight against racist rule. He has retired from public life.

Bizos, Mandela’s lawyer, grew nostalgic as he toured Chancellor House Wednesday. The door of what had been Mandela’s office was decorated with a photo of a young Mandela, books and files under his arm, standing at the window of Tambo’s adjoining office.

Bizos said that when the building was still a ruin, he had visited with a British journalist has asked for a tour of Mandela sites.

“I’m sorry to say that he actually took some pleasure in the mess that there was. And he angered me by saying, ‘Is this the new South Africa?”’ Bizos said. “I’m going to try to bring him back here.”