UN under secretary general for management Alicia Barcena told reporters that the contract was signed early Friday with the US subsidiary of Skanska at an initial cost of seven million dollars.
A statement on the deal put the total anticipated value of the work at around one billion dollars.
UN officials explained that the 1.9 billion-dollar price tag for the renovation project include other costs to cover contingency funding, inflation, leasing and fees for consultants and architects.
The overhaul, which is to begin in the first half of next year, is expected to be completed by 2014.
"Skanska USA Buildings was selected in a competitive bidding process involving an extensive technical, contractual and commercial evaluation by the United Nations and independent outside senior industry experts," Barcena said.
She called the deal "a major step in making the UN headquarters a safer and healthier, more secure and more energy efficient place for all of us who work here", UN staff, diplomats, journalists and the hundreds of thousands of tourists who visit every year.
Last December, the UN General Assembly endorsed the seven-year renovation known as the "Capital Master Plan," restating "its serious concern at the hazards, risks and deficiencies of the current conditions of the UN headquarters building."
The plan involves a massive overhaul of the six-building UN headquarters complex, a major tourist attraction, to bring it into conformity with New York City health and safety codes while improving security.
The main UN headquarters buildings were built between 1949 and 1951 and have not been overhauled since.
The complex has a leaking roof, is riddled with asbestos and lacks fire detectors, a sprinkler system and other emergency safety devices.
Under the plan, 10 of the main compound's 39 floors would be evacuated at a time to allow the structure to be renovated in four stages.
Beginning next year, a temporary conference building is to be erected on the UN complex's north lawn to accommodate the General Assembly.
And 10 office floors are to be leased in central Manhattan to accommodate 750 headquarters staff while additional space is to be leased on Long Island to temporarily house the UN library and about 250 employees.
During the preconstruction phase, Skanska will assess existing design drawings, seek out potential means to save costs and provide detailed cost estimates, Barcena said.
After receiving the guaranteed maximum price for construction, the UN will make a decision on awarding the final construction management deal for the project, she added.
Barcena underscored the importance of improving safety, including by upgrading signs throughout the headquarters complex, installing additional emergency lighting and having fire guards patrolling the secretariat building.
"This will be an enormous project that will touch every one of us who work in this building for many years to come," she noted.
Introducing the newly appointed executive director of the renovation project, Michael Adlerstein of the United States, Barcena said he would take up his post on Monday.
Adlerstein has worked on renovation projects ranging from the New York Botanical Garden and Ellis Island to India's famed Taj Mahal.