Paxton, 24, went 8-11 with a 4.45 ERA in 28 appearances [26 starts] for Triple-A Tacoma. He struck out 131, averaging 8.09 strikeouts per nine innings.
"It was been an up-and-down season for me," Paxton said. "At the beginning of the year, I kind of struggled while learning the league. I got to know the batters better, got more confidence and rattled off some good outings. Then, I had a couple of rough ones, but I feel like I finished strong. I'm coming in feeling pretty good about my stuff and where I am."
Paxton said that, through his experience in Triple-A, he has developed confidence in throwing offspeed pitches behind in the count. He has mixed a cutter into his repertoire and is enthused about an improving changeup.
"It's great to get those 'firsts' out of the way," manager Eric Wedge said. "He has worked hard to get here and deserves to be here. You get a chance to compete at the highest level and work off where you are from there."
Besides Paxson, the Tuesday callups following the completion of Tacoma's season were right-hander Chance Ruffin, right-hander Hector Noesi and infielder Carlos Triunfel. Tacoma hitting coach Howard Johnson was also added to the Mariners' staff for September.
Ruffin, 24, returns to the Major Leagues for the first time since 2011. He split the season between Double-A Jackson and Tacoma, combining to go 5-6 with a 3.91 ERA.
Noesi, 26, is beginning his fourth stint with Seattle this season. In nine appearances with the Mariners, he was 0-1 with a 5.64 ERA. Both Ruffin and Noesi will be expected to help bolster the bullpen. Triunfel, 23, appeared in nine games with the Mariners over two stints before the All-Star break, going 2-for-24 at the plate.
The active roster now includes 32 players. With the addition of Paxton, the 40-man roster is full. Right-hander Stephen Pryor is on the 60-day disabled list and first baseman/designated hitter Jesus Montero is on the restricted list.