Attorney General Edwin Meese III has picked a new chief of staff to replace the departing John M. Richardson Jr., tapping Mark Levin, who had been his special counsel since last January. Levin, 30, a graduate of Temple University School of Law, was an associate deputy attorney general from March 1985 to July 1986, moved to the Interior Department to be deputy solicitor from July 1986 to January 1987, then returned to Justice to work for Meese.

Meese has also selected a new executive assistant, Steve A. Mathews, who had been detailed last year to serve as his assistant on Iran-contra matters. Mathews, a 32-year-old Yale Law School graduate, was deputy assistant attorney general in the Office of Legal Policy, handling judicial selection, and before that was special counsel to Assistant Attorney General William Bradford Reynolds.

Nomination Stalls . . .

Agriculture Secretary Richard E. Lyng's choice for undersecretary for international relations and commodity affairs, one of the USDA's top policy positions, apparently has run into trouble in the clearance process. Lyng refused to elaborate, but he acknowledged last week that problems have stalled the potential nomination of Thomas O. Kay.

The key position, open since last August, has been filled in the interim by Richard W. Goldberg, the deputy undersecretary. Kay has remained as head of the Foreign Agricultural Service since Lyng let it be known that he was his choice to succeed Daniel G. Amstutz. Amstutz became the chief U.S. agricultural negotiator at the General Agreement on Tariffs and Trade talks in Geneva.

Larry Wilson, 48, has been appointed director of the Office of Financial Management for USDA. Wilson formerly was deputy director. He was promoted two weeks ago.

Agricultural Research Service Administrator T.B. Kinney Jr. has announced joint winners for the service's 1987 distinguished scientist of the year, the first time in the competition's six-year history that has happened. The winners are Richard F. Wilson, a soybean researcher in Raleigh, N.C., and Thomas J. Sexton, an avian physiologist at Beltsville.

Joining Lawyers' Legions . . .

FAA chief counsel E. Tazewell Ellett has left the agency to rejoin the legions of Washington lawyers. Effective last week, Ellett returned as a partner in his old law firm, Hogan and Harston. Ellett left the firm in 1982 to become special assistant to Donald D. Engen, then on the National Transportation Safety Board. He moved up to the FAA when Engen became FAA administrator.

Ex-Bear on Drug Panel . . .

President Reagan has named former Chicago Bears star Gale Sayers as a member of the new White House Conference for a Drug Free America. Also among the seven initial appointments is Edward Fritts, president of the National Association of Broadcasters.