Former city administrator Julian R. Dugas has resigned from the District of Columbia government - after 16 years as an often obscure government attorney, 10 more years as the powerful confidant of former Mayor Walter E. Washington and six months as an unwanted holdover in the administration of Mayor Marion Barry.

Dugas, 61, submitted his resignation to administration officials last week. It will become effective July 1 - in time to prevent Dugas, a Howard University Law School graduate, from being subject to a new federal ethics law that sharply curtails activities of lawyers who leave government and enter private practice.

City personnel officials couldn't say yesterday what Dugas would do after retirement. Since late January, after being pushed aside in the Barry administration, he has served as a special assistant to the president of Howard University in an arrangement in which city government pays about half of Dugas's $47,500-a-year salary.

During his tenure in the Washington administration, Dugas developed a reputation as an iron-handed czar over the city's vital licenses and permits division. He was also chairman of the city's Alcoholic Beverage Control Board and then-Mayor Washington's principal liaison with the city's politically influential business community.

After Barry defeated Washington in last year's Democratic primary, the lame-duck mayor reappointed Dugas ABC board chairman until 1981. Dugas has already arranged to resume his one-time post as director of the D.C. Department of Licenses, Investigations and Inspections after Washington left office.

In January, Barry reassigned Dugas to the city office of personnel and replaced him with Robert Lewis. After that, the arrangement with Howard, similar to plans used elsewhere in civil service, was put into effect.

In resigning last week, Dugas Also left the ABC board. Florence L. Tate, Mayor Barry's press secretary, announced yesterday that Larry C. Williams - a lawyer, lobbyist and early political supporter of Barry - would be nominated to replace Dugas on the board.

City personnel officials said Dugas would receive about $22,000 a year in retirement benefits. He was able to retire under optional retirement provisions that allow one to leave after turning 60 and serving at least 25 years in government.

Jose Gutierrez, acting director of the D.C. Office of Personnel, said that had Dugas not quit effective July 1 the new law would have kept him from representing clients before D.C. government agencies for at least two years and would have barred him for five years from working with firms that had business dealings with the city government.