It was a year ago in October that Winnie Kelly cruised down to the Mayflower Hotel and became convinced, finally, that the business elite had accepted him as "a legitmate Washington area personality."

He was sitting there on the dais at the annual Metropolitan. Washington Board of Trade luncheon when the realization came to him in a most currious way - through the jokes that these important and influential people were telling about him.

"The jokes weren't belittling the way they once were," he recalled. "I got the feeling that they were joking with me, that I was being accepted as an equal, as one of the gang."

To Winfield M. Kelly Jr., the short and peppery country executive of Prince George's County, this was not just a reflection his own upward mobility. Here was proof, he thought, that his county - the one he was born in, the one he made a million dollars in, the one he now governed - no longer was dismissed as a cultural embarrassment and a convenient repository for the execesses and the problems of urban Washington.

Few things mean more to Kelly than how Prince George's County is perceived by the corporate leaders and opinion-makers around the nation's capital.

He believes that the condescending attitude of this establishment was partly responsible for making "P.G." somewhat of a regional joke in the 30 years after World War II. It was this establishment, he thinks, that decided the best way to improve Washington was to tear down the slums and let poor people relocate by the thousands in apartments on the other side of the District line, in places like Belford Towers, a Board-of-Trade-sponsored fortress on New Hampshire Avenue.

And then, the dead accomplished, it was these same influential people, few of whom would think of living in Prince George's themselves, who watched most scornfully as the county staggered and fell under the weight of zoning scandals and racial conflicts.

A few years ago, Kelly was bitter and defensive when he talked about this subject. Today, as he completes his four-year term as the executive - the mayor, in effect - of the largest county in Maryland, he is less strident.

"The perceptions have changed dramatically," Kelly said the other day. "I think we're getting a fair shake now. The word has gone out in the business world that Prince George's County has stabilized."

Kelly, a man whose life has been shaped by dollars and cents, is rarely more animated than when he speaks about the business world. His face betrays the same passion as when he bites into a pizza with everything on it at Ledo's Restaurant.

"Economic development," Kelly said recently, without hesitation, when asked what gave him the most pleasure in his job.

"I just love it. I get a tremendous thrill when I drive down one of our highways and look out the window at a vacant piece of land and realize that someday soon there's going to be a clean corporate office standing right there, yielding high tax dollars. I love it . . . love it."

This is a subject that seems less than earthshaking to many of the elderly and blue-collar workers and welfare recipients who comprise such a large part of his county's 700,000 population.

Many of them wonder why Kelly spends so much of his time and so much of his money - hundreds of thousands of dollars in advertising alone - in his effort to make Prince George's appealing to corporate America. They ask the question that an unemployed black recently posed in Fairmount Heights.

"I see Mr. Kelly doing everthing he can to make it easier for business," this man said. "But where's the interest for people like me? I don't see it."

Kelly and his advisers are aware of such complaints. "There's an impression in some places that we care only about the rich," said one of his young aides. John A. Lally. "Sure, we've working to get corporations to move here and upwardly mobile executives to live here, but that's not all there is. It's just that we still have an imbalance and need more rich people to pay taxes to support people who need it."

It was the growth in corporate tax revenues, said Kelly, that helped him cut the property tax bills for most county homeowners this year, for the first time in history.

Kelly believes that the white-collar, college-educated residents of his county agree with his approach and appreciate what he has tried to do. It is these voters - from the middle and upper-middle classes; from Bowie and Greenbelt and Laurel and Clinton - that he is counting on most heavily in his re-election bid next week against an extremely popular Republican opponent, former Rep. Lawrence J. Hogan.

It might appear paradoxical that Kelly, a Democrat for all of his 42 years, would be spurned by his party's traditional blue-collar constituency and accepted by the well-to-do, but social and ideological issues have rarely divided along party lines in Prince George's County.

Kelly said he became a Democrat because his parents were Democrats, not because his friends were Democrats, not because the party represented much of anything to him.

For the first 3 years of his life, in fact, politics did not mean much of truckdriver. He was too busy crafting one of 14 children of a Brentwood.

For the first 30 years of his life, in anything to Winfield M. Kelly Jr., his own rags-to-riches story - from gas station attendant to grocery clerk to lunchwagon driver to restaurant owner to millionaire executive - to consider a public career.

After he had made his fortune, politicians were more interested in Kelly than he was in them. His first overture to run for office came from George P. Mahoney, the ultra-conservative Democratic gubernatorial candidate who asked Kelly to join his ticket in 1966. Kelly said that Mahoney's motive was simple. "He wanted my money. He said it would take about $300,000 to get along with him."

It was a desire to represent the business interests, not to reform what was then a scandal-riddled county government, that brought Kelly to run for the County Commission in 1970. He ran on a slate with the incumbent commissioners and was one of only three people on that slate to survive a reform ticket created by two emerging Democratic powers in the county - lawyer Peter F. O'Malley and state Sen. Steny H. Hoyer.

Kelly was elected chairman of the commission and quickly developed a reputation as a savvy political operator who could be counted on to challenge almost everything that Republican County Executive William Gullett said or did. He actually began running against Gullett in 1972, after only two years in the political game.

There were many Democrats more popular, powerful and experienced than Kelly at the time, but none with as much luck. Hoyer and Councliman Francis Francois and others let Kelly go unchallenged in his primary campaign for county executive because they considered Gullett unbeatable. So did Kelly, privately.

"There was no way in the world I was going to beat the guy, he was a smooth operator," said Kelly. "But then he just went and lost it for himself. He slashed the hell out of the school budget and started talking about how calss size had nothing to do with quality of education. It was like watching an opponent walk into the middle of Beltway.

Kelly surprised himself, and most of his Democratic colleagues, by defeating Gullett. Suddenly, all of the problems of Prince George's County were his responsibility. And, unlike Gullett, he had no one to blame if he could not solve them - every County state senator was a member of Kelly's party.

The high-profile Kelly has been damned so often over the past four years that he is the underdog in his re-election bid - despite a $250,000 campaign treasury and 3-1 Democratic registration in the county.

His detractors include apartment tenants, who for two years were forced to pay a special county surcharge which Kelly now concedes was "a disaster and a mistake," independent Democrats who have sought for years to loosen the grip of the party's dominant faction, now led by Kelly; social service activists who believe the Kelly administration has given them a low priority; and rank-and-file police officers who believe that Kelly has gone too fast in his effort to integrate the predominantly white police force.