He was a law clerk for Judge James H. Taylor, Seventh Judicial Circuit of Maryland from 1973 to 1974.

He was in private practice in Hyattsville, Maryland and Beltsville, Maryland from 1974 to 1986, serving as a municipal attorney for Fairmount Heights from 1975 to 1987, and as a substitute juvenile master, Prince George's County Circuit Court (part-time) from 1976 to 1977.

He was a Special counsel and hearing examiner, Prince George's County Board of Education (part-time) from 1978 to 1987.

Alexander Williams Jr. was the first African American elected to the office of State's Attorney in Prince George's County, Maryland.

On August 6, 1993, President Bill Clinton nominated Williams to a seat on the United States District Court for the District of Maryland vacated by Judge Norman P. Ramsey.