Mark was traded to Houston from Minnesota, where he had been a very mediocre reliever, for a player to be named later in December of 1988. That PTBNL ended up being minor league reliever Todd McLure, who spent one season in the Minnesota farm system before his professional career was over, so Houston basically got Mark for nothing.
Houston decided to turn him into a starter and his career blossomed with the Astros. During his tenure with the team, Mark went 52-30 with a 3.34 ERA. He also hit is only two career home runs while playing for the Astros, one his first season in Houston and the other his last. The best year of Mark's career came in his final season with Houston, 1993, as he went 18-4 with a 2.77 ERA, led NL pitchers in win percentage (.818) and finished 6th in Cy Young voting.
While Mark obviously had several good outings for the Astros, two in particular stood out. July 11th, 1991 Mark threw 10 shutout innings while giving up six hits and one walk with seven strikeouts in an eleven inning 1-0 win over the Phillies. He even went 1-3 at the plate with a double that game.
Another of Mark's best performances came September 21st, 1993 against the Giants. Mark went the distance for a shutout in the 6-0 victory. He allowed just three hits and one walk with six strikeouts. Again, Mark had a good day hitting as well, going 2-3 with a run scored and an RBI.
Mark became a free agent after that 1993 season, eventually signing with those same San Francisco Giants. As compensation for losing Mark, the Astros received the Giants 1st round draft pick in 1994, as well as a compensation pick between the 1st and 2nd rounds. With the first compensation pick the Astros drafted pitcher Scott Elarton. With the sandwich pick Houston selected infielder Russ Johnson.