Mark was born July 21st, 1959 in Corpus Christi, Texas. He was originally drafted by the Kansas City Royals in the 12th round of the 1981 amateur draft. He did not sign with the club. He was drafted again the following year in the 1982 draft by the San Diego Padres in the 4th round.
He played in the Padres minor league system before being traded to the Baltimore Orioles with Terry Kennedy for Storm Davis in October, 1986. Mark was called up to the majors the following year at the age of 27. He played in his first major league game on April 8, 1987 taking the loss 6-4 against the Texas Rangers. Williamson pitched 1.2 innings giving up 2 runs on 2 hits and 2 walks with no strike outs. He went on to finish the year with a record of 8-9 with a 4.03 ERA.
Mark had his best MLB run from 1989 to 1990. In 1989 he finished the season with a 10-5 record and a 2.93 ERA. In 1990 he came back and ended the season with a 8-2 record with a 2.21 ERA.
Mark played his entire MLB career with the Orioles from 1987 - 1994, finishing his career with a record of 46 - 35, an ERA of 3.86, and 397 strike outs and 21 saves. The highlight of his career occured on July 13, 1991 when he pitched a combined no hitter with Bob Milacki, Mike Flanagan, and Gregg Olson against the Oakland A's.