D.C. City Administrator Thomas M. Downs confirmed yesterday that he will leave the D.C. government late next month to become head of a Metropolitan Transit Authority subsidiary in New York City.

Downs, who has held the titles of deputy mayor for operations and city administrator since May 1983 -- longer than any previous city administrator -- will become president of the Triborough Bridge and Tunnel Authority.

Triborough, with 32,000 employes, has a $200 million annual budget and generates $500 million in revenue for mass transit in New York through the operation of seven bridges and two tunnels.

It had long been rumored that Downs, 44, was ready to change jobs. He will receive $115,000 a year in New York, an increase of about $35,000 over his salary as chief of the day-to-day operations of the D.C. government.

Downs said the position would give him a chance to continue public service and to play a role in an $8 billion capital improvement program aimed at modernizing public transit in New York.

"It's time for a change for me," Downs said yesterday afternoon after he and Mayor Marion Barry confirmed a report in The Washington Post that Downs would resign to take the New York position. In a brief interview Downs said he informed Barry three weeks ago that he may take the New York offer.

Barry, who is facing several critical vacancies in his administration, issued a brief statement in which he praised Downs' work and promised that he would "announce a successor to Downs soon."

Downs said he would play a limited role in attempting to name his successor. "It's a very personal choice {for the mayor} -- it's about chemistry, trust . . . vision," Downs said.

The city administrator, who has a son who is in his senior year at a D.C. public high school, said he will commute to his New York job until the end of the school term in the spring.

"As I have said many times before," Barry said in his statement, "I have always tried to attract the most qualified individuals to work for the residents of the District of Columbia. The fact that several of my top aides have been lured away by private industry and other governments is further testimony to my claim that we have the best and the brightest working in the Barry administration."

D.C. Council Chairman David A. Clarke, who is attending the National League of Cities meeting in Las Vegas, issued a statement praising Downs.

"I have a lot of respect for Downs, although we disagreed on some issues," Clarke said through his spokesman Mike Davis. Clarke added that he hopes the administration "will move quickly to fill this important post."