Offseason
- November 1, 1979: Ruppert Jones and Jim Lewis were traded by the Mariners to the New York Yankees for Jim Beattie, Rick Anderson, Juan Beníquez and Jerry Narron.
- December 6, 1979: Rafael Vásquez, Rob Pietroburgo (minors) and a player to be named later were traded by the Mariners to the Cleveland Indians for Ted Cox. The Mariners completed the deal by sending Larry Anderson to the Indians on March 29, 1980.
- December 20, 1979: Willie Horton was signed as a free agent by the Mariners.
- January 11, 1980: 1980 Major League Baseball Draft (Secondary Phase)
- Bill Mooneyham was drafted by the Mariners in the 1st round (13th pick), but did not sign.
- Dan Firova was drafted by the Mariners in the 2nd round.
Read more about this topic: 1980 Seattle Mariners Season