The National Journal, the Washington-based weekly magazine of politics and government known for its reporting on how Washington works, has been sold to Times Mirror Corp. for an undisclosed price.

The Journal is the first Washington publication to be owned by Times Mirror Co. of Los Angeles, which has bureau offices here for five of the newspapers it publishes -- the Los Angeles Times, The Dallas Times Herald, Newsday, The Denver Post and The Hartford Courant.

Anthony C. Stout, chairman and one of the founders of the 17-year-old magazine, owned 80 percent of The Journal. The Washington Post Co. owned the remaining 20 percent, which it purchased in 1983 when Stout approached the company about a loan. Although none of those involved would disclose what Times Mirror paid for the magazine, sources close to the negotiations said they believe the price was between $12 million and $15 million.

There are no immediate plans for changes in either the magazine or its staff of about 40, said David Laventhol, senior vice president for Times Mirror's eastern newspapers division. Laventhol, who will become president of Times Mirror in January, met with The Journal's staff on Friday to assure them of that, he said.

"We think this is a publication of unusual quality and excellence, with unusual readership because of the way it covers the nuts and bolts of Washington," said Laventhol. "We think the whole staff is outstanding . . . the leadership is going to stay and the staff is going to stay.

Laventhol said he did not expect changes in what is perhaps one of the most recognized aspects of the magazine -- its price. The cost of the magazine, which is not sold on newsstands, is $546 a year for each of the approximately 5,000 subscribers.

However, Washington readers of the magazine say it has a much higher readership than its subscription numbers might indicate, because it often is passed around government offices and bound copies are kept in many of them. The publication is often cited, along with Congressional Quarterly, for its unbiased accounts of government.

Stout, who said he has been asked by Times Mirror to stay on as chairman for at least three years, said yesterday that in September he began looking for a large company to invest in The Journal Inc. The parent company includes The Almanac of American Politics, The Capital Source, a directory of important Washington organizations and their phone numbers, and the Convention Daily, a newspaper published by the Journal during the 1984 presidential conventions.

The Washington Post did not pursue increasing its stake in the Journal.

One of the reasons Stout said he went to Times Mirror as a possible investor was that he knew that its president and chief executive officer, Robert F. Erburu, subscribed. "When I was thinking about this, I called him up and said, 'You don't know me, but you read my magazine. . . . How would you like to own it?'

According to its own account, The Journal, which until now has been privately held, lost money in its first dozen years of operation. But in 1982, it began selling advertising and showing a profit, Stout said. However, the parent company needed investors to pay off long-term debts.

Times Mirror is a diversified media company that owns newspapers, television stations, cable television systems and a magazine group. Its sales last year were $2.95 billion.