The District of Columbia's infant mortality rate dropped 16 percent last year to the lowest level since the city began keeping records in 1912, Mayor Marion Barry announced yesterday.

The city recorded 202 deaths of infants under one year of age among 9,386 live births last year, a mortality rate of 21.6 for each 1,000 live births. The previous year, there were 242 deaths among 9,372 births, a mortality rate of 25.8 per 1,000.

An official of the D.C. Department of Human Services said the decrease apparently moved the District from near the top of the infant mortality rate among the nation's 26 largest cities. Saying recent official nationwide figures are not available, he specualted that the District now may be about average among the 26 cities.

Nine days after his inauguration in January 1978, Barry called infant mortality the District's "No. 1 health problem," and ordered health officials to prepare a plan to lower the rate.

Albert P. Russo, then head of the health agency, set a goal of cutting infant mortality by about one-third within a year.The figure released yesterday showed the city achieved about half of Russo's goal.

Barry's announcement came on the eve of a hearing scheduled on infant mortality by the House District Committee.

Barry said the city had taken steps to provide better information, nutrition and parental and hosptial care to expectant mothers. He said he would take steps to assure that every city hospital has an around-the-clock capability to reuscitate and stabilize infants who stop breathing.

The mayor also said he would replace his temporary blue-ribbon panel on infant mortality with a permanent advisory board.

Dr. Frederick Green, chairman of the blue-ribbon panel, told yesterday's news conference that much of the improvement in infant mortality statistics could be credited to greater attention by public and private hospitals to the problem.

There are, Green said, "no simplistic solutions."

Barry, as the newly inaugurated mayor, was spurred to act by a report that the District's infant mortality rate in 1976 was the highest among major cities in the nation and that it increased in 1977 by 9.6 percent while the national rate dropped 11.9 percent.

Actually, the city rate peaked at 28.9 deaths per 1,000 live births in 1970, dropping to a 25.5 rate by 1973, ad then rising again to 28.6 in 1975.

In 1976 it was 24.9, and in 1977 it rose to 27.3.