Fairfax County Supervisor Audrey Moore overwhelmingly defeated County Board of Supervisors Chairman John F. Herrity yesterday, carrying fellow Democrats on her coattails and signaling a new, slower-growth era for the Washington area's most populous and prosperous jurisdiction.

Two of the board's prodevelopment Republican incumbents, T. Farrell Egge of Mount Vernon District and Nancy K. Falck of Dranesville District, were defeated. Democrats, who three years ago lost control of the board for the first time in this century, recaptured it and will hold a 7-to-2 majority.

Five other incumbent supervisors were either unopposed or won reelection. They are Republicans Thomas M. Davis III (Mason) and Elaine N. McConnell (Springfield) and Democrats Katherine K. Hanley (Providence), Martha Pennino (Centreville) and Joseph Alexander (Lee). In the only race for an open seat, Democrat Sharon Bulova, Moore's former aide, easily defeated Republican D. Patrick Mullins to fill the Annandale District seat Moore is vacating.

With 167 of the county's 168 precincts reporting, Moore had 58 percent of the vote, Herrity had 37 percent, and independents James S. Morris Jr. and Robert T. (Terry) Robarge had a total of 5 percent. The unofficial results were Moore, 107,226; Herrity, 67,933; Morris, 5,411, and Robarge, 2,941.

Moore, 58, a Democratic environmentalist, for years has been a lonely voice on the board advocating more stringent controls of the county's explosive development, which she blamed for clogging the road system with traffic. She defeated Republican Herrity by huge margins across the 399-square-mile county and captured traditional Republican strongholds, including Herrity's home Springfield District.

Herrity, 55, telephoned Moore at about 10:05 last night to concede. According to Moore, Herrity said: "You've won, Audrey. Congratulations to you and the rest of the board. It's your job now to run the county."

She replied: "Thank you, Jack. You worked hard and ran a good race. You've done a lot of good for Fairfax County."

Five minutes later, Herrity appeared before his supporters and television cameras to concede. "Tonight the voters in their wisdom have decided to let somebody else" be the chairman. "I accept that judgment of the voters" with the knowledge "that the county has never been in greater shape from either a financial or a quality-of-life standpoint."

Moore, in her televised victory statement, said: "The voters in Fairfax want to balance development and get the traffic moving. And they want us Democrats to do the job."

"I think what you're seeing is a tidal wave," acknowledged Michael E. Murphy, Herrity's chief media consultant. "It's not a Jack Herrity thing at all. It's the whole board; it's a tidal wave against development."

The vote was a milestone for Fairfax and its 710,000 residents. For more than a decade the county's policies have been dominated by Herrity and his probusiness allies, who launched an economic boom that transformed the Northern Virginia suburbs from sleepy bedroom communities to an economic engine. Herrity has been the steward of the boom; Moore has frequently opposed it.

Herrity, a tough-talking Irish pol who became one of the Washington area's best-known local officials, was turned out of office after 12 years as the county's top elected official. His political future is unclear.

On an unseasonably warm, almost muggy day, turnout was heavy at 48 percent of registered voters -- closer to the level expected in a gubernatorial election year than that of a local election year.

Herrity, a member of the board since 1972 and its chairman since 1976, launched what many consider to be Fairfax County's modern era. With the help of a probusiness majority on the board and the support of the county's budding corporate community, he led Fairfax to its current economic prosperity.

Moore, who also first took office in 1972, has been the most vocal and persistent critic of that fast transformation. To the frequent irritation of real estate developers as well as her colleagues, she has consistently sought to impose controls on the county's growth, even after state courts came down on the side of property owners.

In addition to the Herrity-Moore race:In Annandale District, Bulova, a 39-year-old civic activist, defeated Mullins by better than a 3-to-2 margin. In Centreville District, Pennino, a 20-year incumbent who is the board's vice chairman, defeated Republican Linda Douglas by nearly a 3-to-2 margin. In Dranesville District, Democrat Lilla D. Richards, 48, a McLean civic activist, defeated incumbent Falck by less than 1 percent of the votes cast, with independent candidate Robert L. Thoburn, a conservative, apparently siphoning off much of Falck's support. In Lee District, Alexander was unopposed. In Mason District, Davis, 38, a lawyer, trounced independent candidate Daniel Belsole by better than an 8-to-1 margin. In Mount Vernon District, Democrat Gerald W. Hyland, 51, a lawyer, defeated Egge, 57 percent to 43 percent. In Providence District, Hanley, 44, who won a board seat in a special election last year, defeated Republican Myron Smith, 62 percent to 38 percent. In Springfield District, McConnell, 60, who runs a special education school, defeated Democrat Toni M. Carney by about a 5-to-4 margin with independent Thomas E. Giska drawing about 5 percent of the vote.

Moore's day began at 6 a.m. when she cast her vote in the glare of television cameras at her home Chapel precinct. She spent the remainder of the day talking to voters at eight precincts, from Herndon and Vienna in western Fairfax to Springfield -- Herrity's home turf in the southern part of the county.

Herrity began the day with a 6:30 a.m. jog through his neighborhood, and by midafternoon he had visited more than half a dozen polling places across the county.

At a polling place in Vienna, Glennah Moy, a claims examiner for a local insurance firm and 15-year county resident, said her vote for Moore was a vote against Herrity. "I've had it with uncontrolled growth," she said. "I'm not sure she offers a solution, but she offers a change. I'm willing to gamble on a change at this point."

But in Mount Vernon District's Stratford precinct, Frank Norton, a retired telephone technician, said he voted for Herrity as he has in the past. He said he disagreed with Moore's notion of slowing development. "You can't stop growth," he said. "That's where we get our money from."

Herrity never led Moore in the public opinion polls. As early as January -- three months before Moore announced her candidacy and well before Democratic Party leaders were certain she would run -- a newspaper survey showed Herrity trailing Moore by 7 to 8 percentage points. By early autumn, polls showed that Moore had increased her lead.

In part, that early margin stemmed from Herrity's misdemeanor conviction in August 1986, when a Circuit Court judge in Fairfax found that he had violated a state law by failing to disclose his business partnership with a developer before voting on the developer's land use application.

That made Herrity vulnerable. He seemed to compound the damage this year with a number of driving infractions, including one ticket July 4 for going 74 miles per hour in a 40 mph zone. Newspapers obtained transcripts of his Virginia driving record that showed him close to losing his license.

Herrity's advisers said the combined effect of those legal problems was to foster an image of the chairman as insensitive to the law. In particular, Herrity and his aides acknowledged, the speeding ticket dealt them a discouraging setback just as their campaign was getting under way.

Thus a 12-year incumbent found himself playing catch-up against an opponent who was a relative stranger to many, if not most, county voters.

In June, several of Herrity's closest aides urged him to heed the public opinion polls and run his campaign as if he were the challenger and the underdog rather than an incumbent who had never lost a local election. They wanted him to attack Moore early and often, to challenge her to debates, to try to force her to make a critical mistake.

But Herrity reaffirmed the strategy that had led him to four consecutive victories in Fairfax. He refused Moore's offer to debate him during the summer; the campaign, he declared, would begin after Labor Day as it always had.

The result was that Herrity went into the final stretch of the campaign with negative ratings from two of every five Fairfax County voters, which meant he had to win overwhelming support from the rest of the electorate.

Apparently deciding that there was little he could do to improve his image with the voters, Herrity designed his advertising strategy to increase Moore's "negatives." Herrity's natural advantage, his aides thought, was that with his large financial edge he could introduce the relatively unknown Moore to voters on his terms before she had a chance to introduce herself.

The thrust of Herrity's advertising and literature took into account the frustration and near-rage that many voters feel about ever-mounting traffic jams, and it attempted to hold Moore responsible for the county's overburdened transportation system. At the same time, Herrity hammered away in campaign appearances at Moore, emphasizing her problems in getting along with board colleagues and suggesting that she would not be able to lead or work with regional leaders.

Herrity's ads slammed Moore for voting against a number of crucial road projects, including the Dulles Toll Road, the completion of I-66 inside the Capital Beltway and the planned cross-county Springfield Bypass. For a time, the commer- cials seemed to work, and Moore slipped in the polls.

But it was difficult for the chairman to distance himself from a problem that festered "on his watch," as Moore repeatedly said.

Meanwhile, Moore's advisers were determined not to let their candidate slip up. They focused on how to counter Herrity's expected 2-to-1 advantage in fund-raising.