The first day J. William Doswell reported for work at the CIA, Director William J. Casey bluntly spelled out his mission.

"Billy," he said, "we're going to be a no-profile agency."

Nearly two years later, the chief of the agency's Office of External Affairs acknowledges he hasn't succeeded, but not because he hasn't tried. The CIA is once again the focus of public controversy, sparked by news reports that it is conducting a covert military operation to overthrow the Sandinista regime in Nicaragua.

At the United Nations, the Cuban ambassador accuses the agency of plotting a foreign invasion much like the Bay of Pigs. On Capitol Hill, members of Congress say the CIA has overstepped its bounds and is violating the law.

But at the CIA's headquarters on 219 secluded acres just off the George Washington Parkway in Langley, Doswell works hard making sure the agency says as little as possible.

As head of the external affairs office, the congenial former statehouse lobbyist from Richmond oversees CIA congressional affairs and relations with the academic community. He is also the chief CIA aide in charge of public information. But he noted, "We're not here to serve the public."

What Doswell does serve, he says, is the "public interest," and to the CIA that means keeping an increasingly tight lid on public access. Each week, Doswell's 18-member public affairs staff logs about 385 telephone calls from the news media and 40 letters from the public.

Under Doswell's orders, the inquiries are handled courteously and efficiently, including the inevitable queries about unidentified flying objects and complaints from citizens that the CIA has bugged their brains. "Absolutely every phone call is returned and every piece of mail is answered," said Doswell. "But we may not tell you anything. Or we may not tell you what you want to hear . . . . As a matter of fact, our most-often-used phrase is 'no comment.' "

To Doswell, it is all part of a broader Reagan administration effort to limit the flow of public information on national security issues substantially. Doswell has been working to draft legislation exempting new areas of CIA activity from coverage under the Freedom of Information Act.

"The administration is engaged in a concerted campaign to increase government secrecy," said Jerry Berman, legislative counsel for the American Civil Liberties Union, who sees the CIA's efforts as part of the pattern.

The agency wasn't always buttoned up so tight. Six years ago, with the CIA reeling from disclosures about its attempts to assassinate foreign leaders and overthrow hostile governments, then-director Adm. Stansfield Turner asked Herbert E. Hetu, his director of public affairs, to map out a public relations campaign to improve the CIA's image.

Hetu, a former Navy captain, made CIA officials available for news background briefings. Organizations such as college alumni groups were brought in for lectures in the agency's bubble-shaped auditorium. There was a plan for Saturday public tours, although this was called off when the CIA concluded that protecting agency secrets would be a logistical nightmare. "We wanted to lift the mystique about the agency," recalled Hetu, who now runs a Washington public relations firm. "We had a mandate to open the agency up and let people know what was going on."

But Casey and Doswell quickly pulled the plug on Hetu's PR offensive. Hetu's Office of Public Affairs was abolished, its staff reduced by one-third and its work assigned to Doswell's section. Visits by outside groups were halted.

"It was hard for me to justify having the Rockville XYZ Rotary Club come into the CIA," said Doswell. "I don't see how that would benefit the agency." The tight-lipped approach is a change for Doswell, 56, a native Virginian who spent most of his life as a small-town newspaper publisher and lobbyist in the state's General Assembly. Among state lawmakers he was known as a charming, self-effacing southern gentleman who managed to work legislative miracles on behalf of restaurant owners, gasoline dealers and other special-interest groups. "He was maybe the most successful lobbyist I ever saw," said State Sen. Richard L. Saslaw, a Fairfax Democrat. "He would never testify on a bill before a committee. But whenever you met him, you instantly liked him."

After the 1980 legislative session, Doswell retired, but said he soon found himself "bored to death." Later that year, John D. Marsh, a retired insurance executive and racehorse owner who had once hired Doswell to push a parimutuel betting bill in Richmond, suggested he go to work for Casey, who had represented Marsh as a lawyer in Washington. A meeting was arranged and, according to Doswell, the two hit it off immediately.

Now, Doswell, a lifelong Democrat who served as campaign coordinator during Mills E. Godwin's first campaign for governor in 1973, professes total loyalty to the Republican CIA director. He has easy access to Casey, whose office is across the hall on the seventh floor of CIA headquarters. He is also a constant, but silent, companion during Casey's frequent trips to defend the agency's policies on Capitol Hill.

"I believe very strongly in operating in an open society," Doswell said. "I think most people in the agency think that way. But I don't think they want to give away the family store."