Jimmy Carter's man in Illinois was in Washington yesterday being sworn in as Jimmy Carter's man in Oslo.

It came almost three years to the day after Louis A. Lerner had shipped a date to play tennis so he could meet then Georgia Gov. Jimmy Carter at an O'Hara Airport hotel.

"I was probably the first Carter supporter in Illinois," said the new U.S. Ambassador to Norway, not immodestly. "Everybody thought I was nuts."

Yesterday, nobody seemed to think the 42-year-old publisher of a chain of suburban Chicago newspapers was nuts as he stepped up to the Vice President of the United States in the Roosevelt Room of the White House to repeat the oath of office. Least of all the Vice President.

"I really enjoy being Vice President, but there is one job I really want," said Walter F. Mondale, whose Norwegian ancestors used to spell the name "Mundal" after the village.

"That's the one I am swearing-in today."

Lerner really wanted it, too - so much, in fact, that after Jimmy Carter won the election Lerner wrote the President-elect to tell him so.

"I said," said Lerner candidly, "that I'd like very much to be ambassador to a Scandinavian country."

Carter, who made a point during the campaign of disavowing any political debts, nevertheless has been paying some off. Named in recent months to other U.S. ambassadorial posts abroad have been several early supporters: Atlanta newspapers publisher Anne Cox Chambers to Belgium, Atlanta attorney Philip H. Also Jr. to Australia, Cleveland contractor Milton A. Wolf to Austria and Cincinnati financier Marvin L. Warner to Switzerland.

New comes Lerner who, addition to - or perhaps despite - his political qualifications (he was first to introduce Carter to Chicago's late Mayor Richard Daley) is not exactly unprepared or unsuited for the Norwegian post.

Conversant in both Danish and Norwegian, he suited on a foundation grant at Roskilde College outside Copenhagen in 1956.57. "I wanted to find out why Copenhagen, with a population of less than one million, had 13 daily nespapers, and American cities, sometimes much larger, are lucky to have one to two."

He also is prepared to grace the walls of the 60-room Victorian mansion in Oslo which Alfred Nobel built for his daughter, and which the United States governments has owned since the early 1920s, with what friends call his "very, very modern art" collection.

"I'm taking all of it," he said yesterday of his 45-piece collection, which includes works by such contemporary artists as Claes Oldenburg, Larry Rivers, Lucas Samaros, Al D'Arcangelo and William Copley.

He and Susie, his wife, are also taking along recipes since Lerner's $18,000-a-year entertainment allowance has to be shared with his deputies in the embassy.

"It's not a lot of dough," said Lerner at a buffet luncheon-reception he tossed at the Hay-Adams Hotel yesterday following the swearing-in. "But one thing the President has said is don't be lavish, to which I say 'amen.'"

It means low-key and casual entertaining ("that's our style anyway") and also that casual acquaintances and hangers-on can stay home.

"The State Department told me that if we entertain Norwegians, the government pays but if we entertain visiting friends, we pay."

His stock in Lerner Home Newspapers having been placed in a trust, Lerner will draw a $550.000-a-year government salary. He says there will be jug wines for a lot of their Norwegian visitors ("not the king, of course, but his ministers") and that in the growing Carter tradition, even some breakfast meetings.

"The Norwegian foreign minister was here a couple of weeks ago and I asked him what he thought of meeting over breakfast. He smiled and said Oh, I love American breakfast. So I think we can afford a few extra eggs for that."

"A casual guy" by inclination, Lerner said there was one thing that bothered him about being an ambassador; and that was how he might be expected to act.

'But the State Department said look, just be yourself and that made me feel a lot better. I always had the idea that the State Department was full of upper-class twits."

And people in striped suits. But the closet Lerner came to that yesterday was striped tie.

Said one friend at Lerner's swearing in: That's the first white shirt I've ever seen him wear."