[5] Two-time Gold Glove winner Adrián Beltré opted for free agency on November 5, 2009, and later signed with the Boston Red Sox.

[11] Ken Griffey Jr. returned to the Mariners with a one-year deal after a successful 2009 season and large number of Seattle fans asking for "one more year.

The Mariners received former Cy Young Award winner Cliff Lee from Philadelphia in exchange for three prospects as a complement to ace Félix Hernández.

[14][15][16][17][18] On December 18, 2009, the Mariners traded pitcher Carlos Silva and cash considerations to the Chicago Cubs for outfielder Milton Bradley, particularly noting his offensive capabilities.

[27] During the trade for Cliff Lee, the Mariners gave up three prospects including pitchers Phillippe Aumont and J. C. Ramírez and outfielder Tyson Gillies.

[8] On December 28, the Mariners signed catcher Josh Bard, and re-signed pitcher Chad Cordero to minor league contracts with invitations to spring training.

[30] The Mariners signed free agent utility player Chris Woodward, who was in their organization during the first half of the 2009 season, to a minor league contract with an invitation to spring training.

[32] Formerly of the Milwaukee Brewers organization, Jesús Colomé signed a minor league contract with an invitation to spring training with the Mariners on February 10.

[36] Ackley was also named the best prospect in the Mariners' organization by Baseball America, followed by outfielder Michael Saunders, catcher Adam Moore, third baseman Alex Liddi, infielder Carlos Triunfel, pitcher Michael Pineda, shortstop Nick Franklin, outfielder Greg Halman, pitcher Dan Cortes, and corner infielder Mario Martínez.

[37] The list of filers included pitchers David Aardsma, Félix Hernández, Brandon League and Mark Lowe, and first baseman Casey Kotchman.

[41] Seattle invited 23 players to spring training including catchers Eliézer Alfonzo, Josh Bard, Steven Baron, Luis Oliveros and Guillermo Quiróz, infielders Tommy Everidge, Brad Nelson, Josh Wilson and Chris Woodward, outfielders Corey Patterson and Mike Wilson, designated hitter Mike Sweeney, and pitchers Jesús Colomé, Chad Cordero, Josh Fields, Nick Hill, Mike Koplove, David Pauley, Yusmeiro Petit, Mauricio Robles, Chris Seddon, Steven Shell and Levale Speigner.

On February 8, newly acquired pitcher Cliff Lee underwent minor surgery to remove a floating bone spur in his left foot.

[46] On April 3, Mariners' minor leaguer James McOwen dislocated his shoulder while attempting to catch a ball hit by Clint Barmes.

[51] First baseman Mike Carp was assigned to minor league camp on March 25, and was informed that he would start the season with the Triple-A Tacoma Rainiers.

[53] Also on March 31, the Mariners assigned pitcher Chad Cordero to minor league camp, and optioned Luke French to the Triple-A Tacoma Rainiers.