Mobil Corp. notified employees at its mid-Manhattan headquarters yesterday that the company had reached an agreement to purchase its building at 150 E. 42d St.

But Rawleigh Warner Jr., chairman of the nation's seventh largest industrial corporation, also said the planned purchase would have "no impact" cn Mobil's plans to move its 800-person domestic marketing and refining division to metropolitan Washington.

Mobil has an option on a 130-acre site bounded by the Capital Beltway, Arlington Roulevard and Gallows Road in Fairfax County. Currently, final details are being worked out on highway access ramps and other technical matters, following zoning approval by the country's board of supervisors last August.

However, Mobil officers also have been studying a plan to move nearly all its 3,500-person headquarters out of Manhattan to the Fairfax site. Although Warner has said he would retain an office in new York in the event of a corporate relocation, New York City representatives have been trying to convince the oil and retail giant (it owns Montgomery Ward) to remain there.

A spokesman for Mobil said last night that yesterday's announcement does not mean that a decision has been made to stay in New York. "No decision on a full move" has been made, he said.

Rather, the Mobil spokesman noted, the company has been interested for several years in acquiring the property. If Mobil does decide to relocate its headquarters here, the move would take place over many years. The company still would need major New York facilities into the 1980s.

Last October, during a critical water shortage in Northern Virginia, Mobil wrote to Fairfax County officials to express concern about future water resources. A county spokesman said last night that "all systems are go . . . there are no problems" with Mobil's planned move here.