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The 1980 Seattle Mariners season was their fourth since the franchise creation, and ended the season finishing seventh in the American LeagueWest with a record of 59–103 (.364).
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.
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.
Regular season
September 30, 1980: While pitching for the Mariners against the Kansas City Royals, Rick Honeycutt taped a thumbtack to his finger to cut the ball. Royals baserunner Willie Wilson spotted the tack from second base. The umpires investigated and not only found the tack, but also a gash in Honeycutt's forehead. Honeycutt was ejected from the game, suspended for 10 games, and fined.
Johnson, Lloyd, and Wolff, Miles, ed., The Encyclopedia of Minor League Baseball, 2nd and 3rd editions. Durham, North Carolina: Baseball America, 1997 and 2007