Marcus D. Williams always thought he would end up behind a saxophone or a bass guitar, not a judge's bench.

While earning honors at Fisk University in Nashville in the early 1970s, Williams built a reputation as a free-lance musician, traveling around the South and playing with some of the biggest names in the business, including Duke Ellington, Quincy Jones and the Billy Taylor Trio.

Then fate intervened: The oil crisis struck, gasoline stations began closing at night and he was unable to find fuel for his trips. Williams "ended up canceling a lot of gigs" and his musical career was over.

He found a new love -- law -- and today he will be sworn in as a judge in Fairfax County's General District Court, becoming the county's first black jurist.

Appointed to the judgeship during the Virginia General Assembly's last session, Williams recently resigned his job as an assistant county attorney in preparation for his new duties at the Judicial Center.

"Right now, I'm just concerned about doing a good job -- trying to find the delicate balance of working through that {heavy} docket efficiently and still giving people their day in court," said Williams, 34, during a recent interview at his home.

"He'll make an excellent judge," said County Attorney David T. Stitt, who as president of the Fairfax Bar Association will present Williams with a black robe during the swearing-in ceremony. "He's the kind of person you want to see going on the bench. He doesn't have a temper, and he doesn't have a big ego."

Sitting recently in his kitchen with his wife Carmen, 38, and their 2 1/2-year-old son Aaron, Williams said he turned his sights from music to the law and to Washington.

After earning a law degree at Catholic University in 1977, he spent a year abroad on fellowship, studying European business law in London and traveling in Africa. After his return, he worked as a prosecutor in the Fairfax commonwealth's attorney's office for two years before heading to the county attorney's office, where he handled contracts, consumer affairs and utility regulation with the State Corporation Commission.

"He's just one of the nicest people who ever walked through this door," said Deputy Commonwealth's Attorney V. Britt Richardson Jr., who was an assistant along with Williams. He also was one of the few blacks who has applied to the office, said Richardson.

Of roughly 200 applicants in the last four years, only about two have been black, he said. Richardson and others cited the fact that few blacks practice in the area as the main reason Fairfax is just getting its first black judge.

Of Williams' days as a prosecutor, Richardson said, "He was very conscientious, not one to shoot from the hip. He has stayed a student of the law . . . and not everyone does."

Asked about his elevation to the bench at the age of 34, Williams laughed and pointed to his head: "I feel old -- I've got gray and everything."

Being considered for the judgeship "was a hair-raising process. The competition was stiff," he said.

To prepare for his new job, Williams went through three weeks of orientation. One of the differences will be relations with colleagues. Being a judge means "losing the closeness you may have had with some lawyers," he said.

Williams, the son of a dentist and a teacher, also was an adjunct professor at George Mason University, teaching graduate and undergraduate courses in the Department of Accounting and Business Legal Studies. He has taught there part time for the last seven years and recently was honored by President George W. Johnson at a reception.

Williams said his grandfather, who is deceased, had a profound impact on his life. He was an academic and a minister with a PhD in philosophy. He was at the University of Chicago in the late '20s.

Just as some who know him are unaware of his love of teaching, many are surprised to learn about his musical past.

"No kidding?" said Richard L. Coffinberger, associate dean of the business school. "What makes him stick out in my mind is that he clearly must enjoy teaching." He said Williams often will phone and inquire: " 'Gee, don't you have any courses for me this semester?' "

In addition to music, Williams said he relaxes by teaching and writing on his home computer. But mostly, he cherishes time spent with his son and wife.

Carmen Williams is an associate professor of dermatology and pathology at George Washington University Medical Center and director of dermatopathology. The couple met on a blind date and were married in 1983.

Williams will be sworn in at 4:30 p.m. today by Barnard F. Jennings, chief judge of the Circuit Court. He will be the eighth sitting judge on the General District Court, which handles traffic and other cases. Jennings testified to the dramatic change that will happen when he dons the black robe.

"Anybody, as soon as they become a judge, it's a different world almost," said Jennings. "There will be a marked change, I'm sure of that."

Williams said he is prepared: "Some people already treat me a little differently. You can tell they're a little uneasy, and that's unfortunate." But he quickly added: "The exception is my office -- I'm probably getting teased more than I've ever been."

And how does Aaron feel about his dad becoming a judge?

"He's oblivious to everything but Big Bird," said Williams. "That is his hero."