Offseason

• November 1, 1979: Ruppert Jones
  Ruppert Jones
  Ruppert Sanderson Jones is a former Major League Baseball outfielder who played for the Kansas City Royals, Seattle Mariners, New York Yankees, San Diego Padres, Detroit Tigers and California Angels...
  
   and Jim Lewis
  Jim Lewis (1980s pitcher)
  James Martin Lewis is a retired Major League Baseball pitcher. He played during four seasons at the major league level for the Seattle Mariners, New York Yankees, and Minnesota Twins. He was signed by the Mariners as an amateur free agent in...
  
   were traded by the Mariners to the New York Yankees
  New York Yankees
  The New York Yankees are a professional baseball team based in the The Bronx, New York. They compete in Major League Baseball in the American League's East Division...
  
   for Jim Beattie
  Jim Beattie
  James Louis Beattie is a former Major League Baseball player and executive who pitched in the major leagues from –....
  
  , Rick Anderson
  Rick Anderson (baseball, born 1953)
  Richard Lee Anderson was a Major League Baseball pitcher. He was born in Inglewood, California and attended high school in Gardena, California....
  
  , Juan Beníquez
  Juan Beníquez
  Juan José Beníquez Torres is a former center fielder in Major League Baseball who played for the Boston Red Sox , Texas Rangers , New York Yankees , Seattle Mariners , California Angels , Baltimore Orioles , Kansas City Royals and Toronto Blue Jays...
  
   and Jerry Narron
  Jerry Narron
  Jerry Austin Narron is a former Major League Baseball catcher and manager. He currently serves as the bench coach of the Milwaukee Brewers. During an 8-year playing career, he played from 1979–1987 for three different teams...
  
  .
• December 6, 1979: Rafael Vásquez
  Rafael Vásquez (baseball)
  Rafael Vásquez Santiago is a former Major League Baseball pitcher. Vásquez pitched in 9 games for the Seattle Mariners in .-Sources:...
  
  , Rob Pietroburgo (minors) and a player to be named later were traded by the Mariners to the Cleveland Indians
  Cleveland Indians
  The Cleveland Indians are a professional baseball team based in Cleveland, Ohio. They are in the Central Division of Major League Baseball's American League. Since , they have played in Progressive Field. The team's spring training facility is in Goodyear, Arizona...
  
   for Ted Cox
  Ted Cox (baseball)
  William Ted Cox is a former shortstop who played in Major League Baseball from through for the Boston Red Sox , Cleveland Indians , Seattle Mariners and Toronto Blue Jays . He batted and threw right-handed.Cox was selected by the Boston Red Sox in the first-round of the 1973 draft ahead of Fred...
  
  . 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
  1980 Major League Baseball Draft
  -First round selections:The following are the first round picks in the 1980 Major League Baseball draft.- Compensation Picks :*William Erickson, was drafted in the second round by the San Francisco Giants, 31st pick overall. *Jeff Horne was drafted in the second round by the Pittsburgh Pirates,...
  
   (Secondary Phase)
  • Bill Mooneyham
    Bill Mooneyham
    William Craig Mooneyham is a former major league baseball pitcher for the Oakland Athletics.Mooneyham attended Merced Junior College. He was originally drafted in in the 1st round . He made his major league debut on April 19, 1986 with Oakland...
    
     was drafted by the Mariners in the 1st round (13th pick), but did not sign.
  • Dan Firova
    Dan Firova
    Daniel Michael Firova is a former Major League Baseball catcher. He caught 17 games: 13 in and 3 in for the Seattle Mariners, and 1 in for the Cleveland Indians. In most of those games, he entered late in the game as a defensive replacement...
    
     was drafted by the Mariners in the 2nd round.

Regular season

• September 30, 1980: While pitching for the Mariners against the Kansas City Royals
  Kansas City Royals
  The Kansas City Royals are a Major League Baseball team based in Kansas City, Missouri. The Royals are a member of the Central Division of Major League Baseball's American League. From 1973 to the present, the Royals have played in Kauffman Stadium...
  
  , Rick Honeycutt
  Rick Honeycutt
  Frederick Wayne "Rick" Honeycutt is the current pitching coach for the Los Angeles Dodgers. Honeycutt was a left-handed pitcher for 21 years from 1977 to 1997. He played with the Seattle Mariners, Texas Rangers, Los Angeles Dodgers, Oakland Athletics, New York Yankees, and the St. Louis Cardinals...
  
   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.

Notable transactions

• April 9, 1980: Dave Heaverlo
  Dave Heaverlo
  David Wallace Heaverlo is a former right-handed Major League Baseball relief pitcher.He attended Moses Lake High School in Moses Lake, Washington, graduating in 1968. He then went on to Central Washington University, in Ellensburg, Washington, where he graduated in 1973...
  
   was selected off waivers by the Mariners from the Oakland Athletics
  Oakland Athletics
  The Oakland Athletics are a Major League Baseball team based in Oakland, California. The Athletics are a member of the Western Division of Major League Baseball's American League. From to the present, the Athletics have played in the O.co Coliseum....
  
  .
• June 14, 1980: Paul Serna
  Paul Serna
  Paul David Serna is a former professional baseball infielder. He played two seasons in Major League Baseball for the Seattle Mariners.Serna was signed as an amateur free agent by Seattle in June of , and by the following September he was in the major leagues. Playing mostly shortstop down the...
  
   was signed by the Mariners as an amateur free agent.
• June 16, 1980: Casey Parsons
  Casey Parsons
  Casey Robert Parsons is a former Major League Baseball outfielder. He played parts of four seasons in the majors, between and , for the Seattle Mariners, Chicago White Sox, and Cleveland Indians. In four seasons, he played in just 63 games, and in less than half of those did he appear in the field...
  
   was purchased by the Mariners from the San Francisco Giants
  San Francisco Giants
  The San Francisco Giants are a Major League Baseball team based in San Francisco, California, playing in the National League West Division....
  
  .

Draft picks

• June 3, 1980: 1980 Major League Baseball Draft
  1980 Major League Baseball Draft
  -First round selections:The following are the first round picks in the 1980 Major League Baseball draft.- Compensation Picks :*William Erickson, was drafted in the second round by the San Francisco Giants, 31st pick overall. *Jeff Horne was drafted in the second round by the Pittsburgh Pirates,...
  
  
  • Darnell Coles
    Darnell Coles
    Darnell Coles is a former Major League Baseball and Nippon Professional Baseball player who played primarily as a third baseman and outfielder from to...
    
     was drafted by the Mariners in the 1st round (6th pick). Player signed June 13, 1980.
  • John Moses
    John Moses (baseball)
    John William Moses , is a retired professional baseball player who played outfield in the Major Leagues from 1982-1992. He later was a 1st Base/Outfield Coach for the Seattle Mariners from 1998–2003 and the Cincinnati Reds in 2006...
    
     was drafted by the Mariners in the 16th round.
  • Ernest Riles
    Ernest Riles
    Ernest Riles is a former shortstop and third baseman in Major League Baseball. From 1985 through 1993, Riles played for the Milwaukee Brewers , San Francisco Giants , Oakland Athletics , Houston Astros and Boston Red Sox...
    
     was drafted by the Mariners in the 21st round, but did not sign.

Roster

Starters by position

Other batters

Starting pitchers

Other pitchers

Relief pitchers

Farm system