Offseason
- December 5, 1978: Enrique Romo, Rick Jones, and Tom McMillan were traded by the Mariners to the Pittsburgh Pirates for Odell Jones, Rafael Vásquez, and Mario Mendoza.
- December 8, 1978: Craig Reynolds was traded by the Mariners to the Houston Astros for Floyd Bannister.
- December 21, 1978: Mario Díaz was signed as an amateur free agent by the Mariners.
- January 27, 1979: Willie Horton was signed as a free agent by the Mariners.
- February 8, 1979: Jim Todd was released by the Mariners.
- February 22, 1979: Mike Davey was purchased by the Mariners from the Atlanta Braves.
Read more about this topic: 1979 Seattle Mariners Season