Mobil Oil Corp. said yesterday it will keep its corporate headquarters in New York City, but will move 1,300 employees to Fairfax County.

The announcement dashed the hopes of Fairfax County officials of luring Mobil's 3,500-person headquarters and its millions in payrolls and taxes to Virgina.

But the 1,300 Mobil workers who will transfer to Fairfax starting in 1980 represent 450 more jobs than Mobil had originally planned to shift to the Washington suburb.

Mobil announced in 1976 that its U.S. Marketing and Refining Division, with 850 employes, would be moved from Manhattan to a 130-acre tract bounded by the Capital Beltway, Arlington Boulevrd and Gallows Road.

That would be the first phase of construction on a corporate campus that was eventually planned to house Mobil's entire headquarters operation, Mobil officials said, although they never officially announced the would move.

The $28-billion-a-year conglomerate said yesterday, however, that only about one-third of its corporate headquarters staff would be moved to Fairfax. "The remaining two-thirds of the work force will stay in New YorK," Mobil sai.

New York Mayor Edward Koch hailed the decision as a victory in the city's fight to end the exodus of jobs.

"If I owned my own car, I'd buy Mobil gas," Koch told the Associated Press. "I'm delighted by their decision to keep their corporate headquarters in New York."

Mobil executives refused to comment on their reasons for chosing New York over Fairfax, but admitted the company was under what one spokesman called "very heavy pressure" not to move out of Manhattan.

Mobil reportedly has been the prime target of a New York City task force, headed by former deputy mayor Osborn Elliott, that was lobbying business to stick with the city.

Last week the company revealed it has agreed to purchase its headquarters building at 150 E. 42nd St. in midtown Manhattan, Mobil spokesman claimed "no decision on a full move" to Fairfax has been made, and assured Virginians that buying the New York building would have "no impact" on the Fairfax situation.

The announcement from Mobil said the company will take title to the Fairfax County tract on Thursday and will begin grading the site by the end of the month.

Two buildings will be built. An eight-story office building will have two wings connected by a cylindrical core housing elevators and building services. An adjacent three-story building will house eating facilities, medical services and a training center.

It will take about two years to build the offices and the first workers will move in about mid 1980, a few months behind Mobil's original schedule.

Although the company raised the number of employes who will be moved into the site by nearly 50 per cent, it did not reveal what operations would join the marketing and refining division in Fairfax.

The company promised its New York workers that anyone who wants to move will be offered the opportunity and said "no further transfers are contemplated" after this move.

A spokesman said it was assumed some workers would not want to leave New York, creating some job openings in Fairfax. He declined to estimate how many local openings might be available.

If Mobil had come to Fairfax, it would have been the largest corporation based in Virginia, surpassing Reynolds Metals of Richmond. Mobil is the seventh largest industrial firm in the nation, encompassing not only its petroleum operations, but also Montgomery Ward and Co., the big retailer, and Container Corporation of America, a major packing firm.

Fairfax officials estimated the originally planned 850-worker development would have produced between $800,000 and $1 million a year in property taxes, as well as pumping millions of dollars of income into the local economy.

There reportedly was intense infighting at Mobil over the decision, and strong opposition from some employees to the move.

The company had said all along that if the headquarters moved, some operations would still have to be maintained in New York. Mobil chairman Rawleigh Warner said he intended to keep his office there, prompting underlings to complain that if the boss could stay in New York, they ought to be able to as well.

Fairfax County development director David Edwards said he believed "a number of internal concerns" at Mobil rather than any problems with Fairfax County prompted the decision.

He said there was "nothing indentifiable or correctible" that could be done to change the decision by Mobil's board of directors. "We have tried to do as a county everything we could to move them."

Edwards expressed disappointment over the decision, nothing that within the past week Mobil executives had talked with Virginia Gov. John Dalton about their plans.

A spokesman for New York's Office of Economic Development said Mobil executives were to meet with that agency this week to discuss their plans.