U.S. Attorney J. Frederick Motz, the federal government's top prosecutor in Maryland since 1981, will be nominated by President Reagan to fill a new seat on the federal court here, the White House announced.

Rumored for months to be the leader among five contenders for the judicial spot, Motz is expected to undergo a Senate confirmation hearing in the next few weeks.

If confirmed, he will join nine active judges on the U.S. District Court in Maryland. The court was increased to 10 members by Congress last summer.

At 42, Motz is a quiet, studious looking man who served as general counsel to the state Republican Party in the late 1970s and was a member of its central committee before being appointed U.S. attorney by Reagan in 1981. He also served as an alternate delegate to the Republican National Convention in Kansas City in 1976.

As U.S. attorney he emphasized the enforcement of drug laws.

A member of the Baltimore law firm of Venable, Baetjer and Howard in the 1970s, Motz first achieved notice in GOP political circles when he headed a committee to revise the state party's constitution and by-laws.

"They were in complete shambles," he said in an interview today. "I guess I did a pretty good job." Thereafter, he became the party's general counsel, its chief legal adviser.

Motz was one of five persons recommended by Sen. Charles McC. Mathias (R-Md.) last October for the $76,000-a-year judgeship. The others were Baltimore U.S. Magistrate Frederic N. Smalkin, Baltimore City Circuit Court Judges Marshall A. Levin and David Ross and Montgomery County lawyer Charles M. Tobin.

Motz, a Baltimore native, lives here with his wife, Diana, and their two children. His wife, also a lawyer, is chief of litigation in the Maryland attorney general's office.