He was drafted by the Toronto Blue Jays in the second round of the 2001 MLB Draft out of Saint Louis School, and was signed on July 3, 2001. He made his major league debut on September 21, 2004 against the New York Yankees as he pitched a solid inning and a third.

Following League's breakout season in 2006, he engaged in strength conditioning during the offseason, and at the behest of the Blue Jays, did not pitch until spring training. As a result, he overdeveloped some shoulder muscles, resulting in a significant drop in his fastball velocity. League was placed on the 60-day disabled list and missed the majority of the 2007 season. Both his velocity and effectiveness recovered the following season.

On December 22, 2009, League was traded to the Seattle Mariners along with minor-league outfielder Johermyn Chávez for pitcher Brandon Morrow. He signed a one-year, $1.08 million contract for 2010. Because of an injury to David Aardsma, League started the 2011 season as the Mariners' closer. He was selected to his first All-Star game as a relief pitcher for the 2011 All-Star game. On June 8, 2012. In a 2011 interview, veteran outfielder Coco Crisp named League as one of the four toughest pitchers he had ever faced. League was one of six Mariners to throw a combined no-hitter against the Los Angeles Dodgers at Safeco Field. After a series of blown saves, League was replaced by Tom Wilhelmsen as closer. In 2 ¹⁄₂ seasons with the Mariners, League had converted a total of 52 saves in 69 chances with a 3.26 ERA

...
Wikipedia