Offseason
- November 9, 1998: Paul Spoljaric was traded by the Mariners to the Philadelphia Phillies for Mark Leiter.
- November 22, 1998: Butch Henry was signed as a Free Agent with the Seattle Mariners.
- December 30, 1998: John Mabry was signed as a free agent by the Mariners.
Read more about this topic: 1999 Seattle Mariners Season