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Thornton has a career ERA of 3.35 in 730 Major League games spread across 12 big league seasons. He's pitched for the Mariners, Nationals, White Sox, Red Sox and Yankees. Thornton spent the past two seasons with Washington.
Thornton, 39, was an All-Star in 2010 with the White Sox and has made his mark with a power arm. He had a 2.18 ERA and 18 holds in 60 games last season with the Nats.
"He's obviously a very established left-handed reliever on a non-roster deal. He brings an element that we don't have," manager Andy Green said. "[He's] right in the thick of the bullpen, and a guy who held lefties to a .484 OPS last year. [He's been] very dominant against lefties and very successful through his career.
"And he's got a chip on his shoulder because he did earn another roster job, but he couldn't find one. So we're excited to have him."
Thornton sounded a little miffed about the offseason and lack of a big league deal.
"It's been a long offseason for me, frustrating at times," Thornton said after his first workout with the team. "But I'm excited to be here. There's a lot of open spots, a lot of jobs [to be] won."
Since 2005, only Chad Qualls (756) and Francisco Rodriguez (726) have appeared in more games than Thornton (711).
Thornton may have lost some fuzz on his fastball (93.5 mph average in 2015), but he can still neutralize left-handed batters, who hit just .198 against him last season.
Thornton, the 22nd overall pick in the 1998 Draft, made his Major League debut in 2004 with the Mariners at the age of 27.
In that game, Thornton tossed four shutout innings of relief against San Diego at Safeco Field. The first hit he allowed was a single to Mark Loretta, who is currently a special assistant with the Padres.
The decision to sign Thornton made sense on a lot of fronts, especially in terms of adding another left-hander in camp. The Padres have Drew Pomeranz, Ryan Buchter and Frank Garces as potential fits from the left side.
Reliever Buddy Baumann, another lefty, remains sidelined with a back injury. He recently returned from San Diego to see team doctors. There's no timetable for his return.
As for Thornton, he's missed the first two weeks of camp, but he has remained in shape by working out at the Fischer Institute in Tempe. Now he's ready to be a Padre.
"They were the team that was most aggressive with their deal with me," Thornton said. "[They made] no promises to me, everyone said that, but said I have a good opportunity to make it."