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.
Seattle Mariners
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 seasons with the Mariners, League had converted a total of 52 saves in 69 chances with a 3.26 ERA
Los Angeles Dodgers
On July 30, 2012, League was traded to the Los Angeles Dodgers for minor leaguers Leon Landry and Logan Bawcom. He became the Dodgers closer in September after Kenley Jansen was sidelined with an irregular heartbeat. In 28 games with the Dodgers, he was 2-1 with a 2.30 ERA and 6 saves. On October 30, the Dodgers re-signed League to a three-year, $22.5 million deal with a vesting option worth $7.5 million for 2016. On June 11, League was demoted from the closer role after blowing his fourth save in 17 chances, and he was replaced by Jansen. He was used primarily in low pressure situations the rest of the season. Overall, in 2013, he was 6-4 with a 5.30 ERA in 58 appearances, with 14 early season saves. The Dodgers, in 2014, used him primarily in middle relief and League was 2–3 with a 2.57 ERA in 63 games. League underwent an MRI during spring training in 2015 which revealed serious shoulder damage, causing him to miss the first couple months of the season. After spending a month rehabbing in the minors, League was designated for assignment on July 2, 2015. He was released on July 10.
League did not play professional baseball at any level in 2016. On January 7, 2017, League signed a minor league contract with the Kansas City Royals that included an invitation to spring training.
After he was released by the Royals, League signed with the New Britain Bees of the independent Atlantic League of Professional Baseball. He became a free agent after the 2017 season.
Pitching style
League's main pitch is a very hard sinker averaging 96 mph. He also has an upper-80s slider that he uses primarily early in the count to right-handed hitters. He also has a splitter that he uses as a strikeout pitch. The splitter has a whiff rate of 55%.
