J. Hamilton Lambert, who has balked for nearly two years at taking the top job in Fairfax County's government, succumbed yesterday and agreed to accept the county executive's position at an annual salary of $65,000 a year.

Lambert, 39, has held the job on an acting basis for 20 months and had repeatedly resisted offers to assume the position on a full-time basis.

The puckish bureaucrat revealed his change of mind yesterday morning at a meeting of the nine-member Fairfax Board of Supervisors that resembled a lovefeast and included predictions that Lambert would overcome any of the problems that haunted his predecessors.

"I certainly appreciate the favor and support that the board has shown in offering this position," said Lambert, after hearing the supervisors lavish praise upon his accomplishments, his leadership capability, and his political expertise. Asked if he would at last reveal why he had previously refused the same offer, Lambert, smiling, would say only, "no."

His unamimous appointment ended an executive search that began 20 months ago, when former county executive Leonard L. Wharton resigned under pressure from supervisors who claimed he was not an assertive leader.

In the course of the search, county leaders spent $10,000 on a professional search firm and interviewed candidates from around the country.

"We made a concerted effort to look for very talented people, and we were forced to conclude that the most talented person was right here under our nose," said Supervisor Audrey Moore (D-Annandale).

Lambert has worked for Fairfax County ever since he graduated from Loudoun County High School in 1959. One of his first actions as the county executive yesterday was to annouce that he plans to propose a major reorganization of the county's public safety system this fall. He declined to reveal details of the plan.

Supervisor Martha V. Pennino (D-'centerville), who headed the board's executive search committee, told the board she was convinced that "no one could better serve Fairfax County" than Lambert, whom she described pubicly as an economic "whiz," and a man who possesses great "political savvy."

Pennino said she had no inside knowledge of the "personal reasons" Lambert has cited twice previously in refusing the executive appointment, most recently last December. She said, however, that the turning point may have been when a majority of the supervisors approached Lambert individually within the last few weeks and asked him to take the job.

"I don't think he was aware he had a unanimous board" before that time, Pennino said.

Unlike the publicly elected executives who head local governments in the Maryland suburbs, Lambert has to reprot to an elected board and must operate without the autonomy granted local governments in that state. All major changes in the Fairfax government must be approved by Lambert, the elected supervisors, and members of a state legislature which has been historically resistant to many of the changes that Northen Virginians have wanted in their local governments.

Fairfax officials were relieved yesterday at the Lambert appointment, and were hopeful it will resolve a long-simmering morale problem in the top echelons of county government. Some county officials complained privately that Fairfax County's government was rudderless, because it lacked a permanent leader at the top.

Lambert is viewed by people who have worked with him as smart, dedicated and hardworking, and he has a reputation for being particularly adept in coping with the many divergent needs of his bosses, the board of supervisors.

Although his formal education ended when he received his high school diploma, Lambert has risen steadily in the county structure since he began as an assistant map draftsman at the age of 18.

In subsequent years he has held positons as divergent as planning technician, management analyst, director of general services and assistant director of public works.

Recently, Lambert has served as the county's liaison with other area governments, and headed the intergovernmnental committee that drew up a compromise agreement with Washington's Blue Plains sewage treatment plant.

In recognition of his role in that agreement, the Metropolitan Washington Council of Governments last year presented him with its Metropolitan Achievement Award for "a major achievement of lasting significance to all of Metropolitan Washington."

It had been suggested earlier that Lambert was reluctant to accept the Fairfax executive post because of a county requrement that the executive reside inside the county. Lambert said yesterday, however, that he was preparing to move within the next four months to Fairfax from Leesburg, where he has lived since childhood. He said the residence question "was not a major issue" in his decision to accept the job.

Pennino said the board had completed in-depth interviews with six finalists last week when the board members decided they preferred Lambert to any of the candidates they had seen. After a meeting with Lambert late in the week, she said, Lambert agreed to take the post.

County officials are hopeful now that Lambert will become more of a leader in county government, introducing initiatives when he thinks necessary rather than waiting for the board's direction to do so. As the acting county executive, they said, Lambert was hesitant to suggest major changes for fear he would be usurping the authority of his successor.

They also expect that Lambert's relationship with the board will likely become a little more stormy. "Up until now it's been good old Jay doing the board a favor," said one official. "Now he's got to come through, and they're bound to get a little more demanding.

"Let's face it -- the engagement is always more fun than the marriage."