Assistant Attorney General Richard K. Willard, the power behind administration efforts to begin drug testing of federal workers, is leaving to join the law firm of Steptoe & Johnson here.

Willard, 39, is the Reagan administration's longest serving political appointee in the Justice Department. He joined the administration in 1981 as special counsel for intelligence policy, and in 1982 was named deputy assistant attorney general. For the last three years he has headed the Civil Division.

In addition to drawing up plans for random drug testing of some federal employees, Willard also helped draft the directive, since dropped, that called for polygraph testing of officials with access to secret information.

Army Official Departs . . .

Undersecretary of the Army James R. Ambrose will leave his post Feb. 27, the Army announced yesterday. Ambrose, 65, who has been on the job since October 1981, had planned to bid his formal adieu at the Pentagon yesterday but the White House asked him to hold off because it had not processed his resignation, according to the Army.

Ambrose, a former vice president of Ford Aerospace and Communications Corp., stood out from the Pentagon's dead-serious bureaucrats by exhibiting country-store friendliness and humor under even the most trying circumstances. One of his biggest struggles, and possibly achievements -- depending on how it turns out -- was guiding the Army's rearmament program.

The biggest project within that program is a new family of helicopters called the LHX, planned as a $60 billion buy. Congressional budget cuts have forced the Army to cut the LHX project in half, with some predicting it will be further restructured after Ambrose leaves.

Award for Publisher . . .

Deputy Defense Secretary William H. Taft IV has presented Washingtonian magazine publisher Philip Merrill the Department of Defense Medal for Distinguished Public Service, the highest Pentagon honor given to civilians. Merrill was cited for his service as a member of the department's policy board and work on the Law of the Sea Treaty.

New Title at Treasury . . .

Robert B. Zoellick, executive secretary to the Treasury Department, has been given the additional title of counselor to Secretary James A. Baker III. The title formalizes a role Zoellick has played as a key Baker adviser on a range of policy issues. Zoellick, a graduate of Harvard's law school and school of government, joined Treasury in July 1985 as an assistant to Richard G. Darman, then deputy secretary.

Adding Inspectors . . .

The fight against fraud, waste and abuse goes on.

The Senate, on an 85-to-0 vote Tuesday, approved a bill to establish inspectors general at the Nuclear Regulatory Commission, the Treasury Department, the Internal Revenue Service, the Office of Personnel Management and the Federal Emergency Management Agency.