"Everyone wants to do business in Quincy,” said one developer at the center of a redevelopment push that has already made the city's downtown nearly unrecognizable to people who grew up here.

QUINCY — The concrete skeleton of what will soon be the city’s tallest building is slowly rising over Quincy Center, the latest evidence of a rapid transformation that has made the city's downtown almost unrecognizable to those who grew up here.

In the last five years, vast parking lots in Quincy Center have given way to multistory parking garages while shabby storefronts and aging office buildings have been replaced by gleaming high-rise apartment buildings.

About one-third of the 3,800 residences that have been built or planned in Quincy since 2015 are in Quincy Center, with many more on the horizon. And alongside the new homes, 19 new restaurants have moved in while retail space spanning tens of thousands of square feet has opened up.

But developers and city officials say they're just getting started.

So far, only 225 of those new downtown condominiums and apartments have opened to tenants, and the promise of a thriving retail economy has yet to be realized in Quincy Center. Nonetheless, after decades of hopes and false starts, Mayor Thomas Koch says he believes Quincy Center is finally on the brink of transformation.

“We are basically creating a new neighborhood here,” the mayor said on a recent rainy afternoon as he stood amid the construction sites of two of the downtown’s largest projects to date.

It wasn't long ago that it seemed Quincy Center might never see the dawn of redevelopment long envisioned by city leaders.

Once dubbed “Shopperstown USA,” Quincy Square's retail district slipped into economic decline in the 1970s and '80s as department stores such as Remick’s, Woolworth and Sears pulled out over the years, unable to compete in a world of malls, outlet centers and big discounters such as Walmart. The area's decline accelerated with the closure in 1986 of the Fore River shipyard, which was already a shadow of what it had been at its height during World War II, when it employed 32,000 people and had a payroll of $110 million, the equivalent of $1.59 billion in today’s dollars.

The first step toward revitalization came in 2007, when Quincy officials declared a 12-block section of downtown "blighted" and created an urban revitalization district there. The designation gave the city broad authority under state law to negotiate with local property owners and private investors to drive redevelopment. It also gave the city a mechanism for acquiring properties, through eminent domain, that it deemed necessary to accomplishing goals for development and infrastructure improvement.

But Quincy's development was still not assured. In 2010, a New York-based developer called Street-Works negotiated a massive $1.6 billion plan to transform 12 downtown blocks, but after four years passed with few of the company's obligations being met, Koch decided to cut ties with the company, leaving the downtown with an uncertain future once again.

“People are quick to demonize Street-Works, but I don’t,” Koch said. “I thank them because they are the ones who really laid the foundation for what we are seeing here today.”

Quincy Center today is a bustling construction zone from one end to the other. Four new residential communities have opened or are about to open in the heart of downtown, and developers say they are keen to take on more projects.

In June, the city signed a deal with FoxRock Properties to bring in the first new major commercial project — a 200,000-square-foot medical complex — that the downtown has seen in decades. It’s a project that Koch says will bring new, high-paying jobs to Quincy and add to the city’s commercial tax base.

Andrian Shapiro, a principal at LBC Boston, one of the first commercial developers to invest in Quincy Center, said he is negotiating with Koch administration officials to buy two more pieces of city-owned land where he wants to build two mixed-use residential and retail towers. LBC Boston is the commercial real estate company behind Nova, a 171-apartment building that is expected to open to tenants soon.

“For Nova, we could barely find a bank to give us a loan,” Shapiro said. “Now the banks are lining up. Everyone wants to do business in Quincy.”

Koch attributes the changing economic climate in Quincy Center to developers including Shapiro, FoxRock Properties and Quincy Mutual Fire Insurance Co., which built the 169-apartment West of Chestnut residence, the first apartment building to open as part of Quincy Center’s redevelopment.

Two other local developers — the Galvin family, whose Cliveden Place condominium project was the second new residence to open downtown, and Peter O’Connell, who is behind the 15-story building now rising in the square — were also instrumental in bringing about big changes, Koch said.

“It’s always difficult to get someone to take that first step, to take on that first project, and ultimately it was a lot of Quincy people who stepped up to make it happen,” Koch said.

But even as development has paved the way for what many consider a brighter and more economically pleasant future in downtown Quincy, it has led to a changing of the guard that has spelled the end for many older Quincy businesses downtown.

When the Koch administration came out with a plan this spring to expand the downtown urban renewal district for a fifth time, it was met with criticism by property owners unsure whether it would leave them in Quincy Center’s future.

Eileen Robertson’s family has owned a strip of storefronts at 1562-1568 Hancock St. for 96 years, but the single-story building is losing its relevance amid the towering high-rises sprouting around it.

“I just feel they want to knock my stores down and put something big up,” she said in April, gesturing at the seven-story Nova apartment building on Hancock street.

Koch’s plan, which city councilors approved in June, identified Robertson’s property and 13 nearby properties as potential acquisitions for redevelopment.

Despite the growing pains, Koch says his administration's revitalization plan is meant to keep development from running rampant across the city by concentrating density in the city’s urban downtown. He said by buying underused properties like Robertson’s, the city can create a better mix of commercial and residential projects.

“Land is a finite resource,” he said.

Follow Erin Tiernan on Twitter @ErinTiernan.