Dodgers general manager Ned Colletti said the club had pursued League, who will be a free agent at the end of the year, at the trading deadline last year and again over the winter but "Seattle wasn't ready to trade him yet."
But after League lost his closer job to Tom Wilhemsen in late May, the Mariners finally parted with the hard-throwing righty. Colletti said the Dodgers would likely use him in a similar role, at least in the short term.
Kenley Jansen has been solid in the closer role since taking over in May, converting 20 of his 25 chances.
"Any time you can add people in that segment of the game [late innings], you do it," Colletti said.
Dodgers manager Don Mattingly said he didn't immediately know how he would use the 29-year-old League, who converted 37 of his 42 saves last season, but felt he would help the bullpen tremendously.
"I know him from the American League and he's dirty," Mattingly said, referring to his days with the New York Yankees, when League pitched for the Toronto Blue Jays. "This is a good arm. If he was in a different spot in his career, they'd really have to think twice about moving him. He still throws 96-97 miles an hour.
"He's one of those guys, as a hitter, that you don't want to see."
The Dodgers traded minor league outfielder Leon Landry and right-handed pitcher Logan Bawcom in the deal.
Colletti said he felt both Landry and Bawcom had a chance to reach the big leagues. Landry has hit .328 with eight homers, 51 RBIs and 20 stolen bases for Rancho Cucamonga, the Dodgers' Class A affiliate. Bawcom, a closer who was recently promoted to Double-A Chattanooga, has a combined 2.03 ERA and 20 saves this year.