D.C. Mayor Marion Barry has chosen Carol B. Thompson, now deputy mayor for economic development, as the District's new city administrator in a top-level realignment that would consolidate all city government operations under her, sources said yesterday.

Thompson, former chief of staff to Barry, would become the first woman to hold the city administrator's post and one of the highest-ranking black women in state and local public administration in the United States.

Thompson, expected to be named as early as today, would succeed Thomas M. Downs, city administrator for nearly five years, who confirmed this week that he will become president of a Metropolitan Transit Authority subsidiary in New York City late next month.

Downs, who had been looking for a job outside government for some time, was told by Barry in August or September to "move on by the end of the year," according to a high-level source in Barry's administration. The source said Downs was "burned out" in the demanding job and that "his judgment was not as good as it used to be."

According to the source, Barry has complained privately that Downs mishandled some of the operational problems that have embarrassed his administration, including snow removal in January and continuing problems with emergency ambulance service.

Downs could not be reached for comment yesterday.

Thompson is highly regarded in her current post but would face significant hurdles as Barry's third city administrator, according to officials who say she may have difficulty meshing her deliberate style with the often crisis-like atmosphere of the city administrator's office.

The quick replacement for Downs -- who has agreed to stay through late January to help with budget issues -- is a sign that Barry intends to deal quickly with several staff vacancies that have hurt morale, according to his aides.

Thompson declined to be interviewed yesterday and referred all questions to the mayor's office.

John C. White, the mayor's press secretary, said Barry has scheduled a 1 p.m. news conference at the Department of Employment Services that will follow a regular monthly Cabinet meeting. Barry aides say the mayor has said he would make several announcements at the meeting. One ranking official said Downs' replacement definitely would be named then.

Sources said Thompson agreed to succeed Downs but sought assurances from the mayor that he would restore institutional power stripped from the position at the start of Barry's second term in 1983 when he installed a system of three equal deputy mayors, one of whom served as city administrator.

Thompson is said to believe that Downs never attained the authority held by his predecessor, Elijah B. Rogers, partly because administrative duties and authority were shared by the three deputies. Rogers was widely seen as a strong-willed city administrator in Barry's first term and was regarded as "speaking for the mayor" in his dealings with city officials. Downs did not have such clear-cut authority.

Under Barry's realignment, according to sources, the city administrator would revert to the traditional number one position under the mayor's office. It was unclear yesterday whether the city administrator also would retain the title of deputy mayor for operations or whether Barry would abolish the deputy mayor posts.

The position of deputy mayor for finance has been vacant since the March 1986 resignation of Alphonse G. Hill, and there was no indication yesterday that Barry has selected a successor to Thompson as deputy mayor of economic development.

The city administrator post is mandated by the District's Home Rule charter while the deputy mayors for finance, economic development and operations were creations of Barry and his former chief political aide, Ivanhoe Donaldson.

Thompson, despite her rapid rise in Barry's government, has rarely been in the news. In 1986, while she was his chief of staff, she took the lead in disclosing that the mayor's travel and other expense records were in disarray. In a letter to The Washington Post, which had requested the records under the Freedom of Information Act, Thompson blamed former administrative aide Robert Robinson for the problems.

Thompson is a Smith College graduate who holds a master's degree in urban policy from New York University. She is a former director of the Department of Consumer and Regulatory Affairs, which she helped create in March 1983.

Thompson also is a former special assistant to the city administrator and a former congressional liaison for the federal Department of Housing and Urban Development.