At age 31 Jeff MacNelly is one of the country's premier public wits. About 350 newspapers buy his editorial cartoons and his off beat strip called "Shoe" is run on 475 comic pages. To hear him tell it, success couldn't have visited a less likely guy.

MacNelly, who grew up on Long Island and attended prep school in Andover, Mass., "I was always scribbling in my notebook. I took notes in the margins. . . Looking back, I shouldn't have gone to college. I was one of those people who needed a hiatus-going to class was kind of an extracurricular activity . . .

"I don't remember worrying a lot. My father used to worry. He'd tel me I was going to hell, and I better get on the sick. And I'd agree with him, and that made him madder.

MacNelly was a mediocre student at Phillips Academy in Andover; he couldn't settle on a major, and he never did get a degree from the University of North Carolina.

His father-formerly a New York ad executive and publisher of the Saturday Evening post, now a portrait painter-despaired for his oldest son.

But today MacNelly, working from a cluttered office at the Richmond News Leader, has won two Pulitzer Prizes for editorial cartooning as well as a number of other honors.

"If he wins any more prizes, we might have to put him in a smaller office," threatens Ross MacKenzie, editor of the News Leader.

From his home base in Virginia's capital, the lanky 6-foot, 4-inch MacNelly works on other projects. This season his name is on two books, a collection of his strips call The Very First Shoe Book , and A Political Bestiary , a humorous look at political jargon by Eugene McCarthy and James J. Kilpatrick. MacNelly illustrated the book.

"He's a born entertainer," says editor MacKenzie of his young star. "One of the secrets of his cartoons is that his first desire is to make you laugh rather than to deliver some profound philosophical point. There was a long period when none of the really good cartoonists were particularly conservative. And Jeff really was the only one who was good and conservative at the same time, though it would be wrong to overemphasize his conservatism."

"Every once in a while," say MacNelly, "I worry that I'm not going for the jugular vein. I think maybe I should be a little meaner. But I think you get more people to listen to you if you poke good-natured fun at people . . . I'm a lot more conservation than most guys you find in my profession. But you in my profession. But you can't afford to be dogmatic and funny at the same time. You can't be the conservative cartoonist. That would be too predictable."

MacNelly's pen respects no political boundary, and while much of his subject matter naturally derives from Washington's follies, he prefers to draw from the safe distance of 120 miles. He avoids the national political social circuit, living quitely in Richmond with his wife and two sons, ages 4 and 6.

MacNelly's office is a mandatory stop for visitors touring the News Leader, and they often gape at the disorder. He kicked that habit of smoking "nasty, foulsmelling cigars," a habit he picked up while drawing for his college newspaper and the local Chapel Hill, N.C., twice-weekly newspaper. For fun he plays handball and "thrashes" at the banjo.

When MacNelly began working for the News Leader in late 1970, he would begin drawing a day before deadline, worry through dinner, work late, and then get up early to dash to the office to pen the finished product. "Then," he says, "I decided to eliminate the 24 hours before. Now I get to work at about 6 a.m., do the cartoon by 9:30. You have to a open, flowing mind, and the best way is not to worry about it."

He won his first Pulitzer 18 months after he began working.

"I remember a New York Times reporter called him to do a brief biography of him, and he asked Jeff about his early life,sex life and political philosophy," says, 'How about a little something about what you do in your spare time?' Of course this was about 15 minutes after he'd won, and Jeff said, 'Well, in my spare time I move furniture for my wife.' Which of course is true. He still does."