The 29-year-old Shelton, who has previous big league experience with the Tigers and Rangers, batted .319 with 12 home runs and 64 RBIs in 76 games for Tacoma this season.
The Mariners signed him as a Minor League free agent in December and invited him to Spring Training. He had a solid camp, batting .460 (23-for-50) with three home runs and 23 RBIs, but there was no room on the roster for the backup first baseman.
He reported to Tacoma and picked up where he left off in Arizona.
"We had just finished playing Portland last night and I was told not to come to the park tomorrow night, but go to Safeco instead. I said, 'You got it.' And here I am."
Though used primarily at third base for the Rainiers, Shelton will get most of his playing time as the DH with the Mariners, according to Wakamatsu.
Asked about the reports he has received regarding Shelton's third-base prowess, the manager said, "Not good. I wouldn't consider [playing him at third] right now. First base is his natural position. He gives us the luxury of a right-hand bat off the bench."
The Mariners created an opening for Shelton by sending left-handed starting pitcher Jason Vargas to Tacoma.
"With the All-Star break beginning on Monday, we will not need a fifth starter until July 25," general manager Jack Zduriencik said.
Making the switch now gives Vargas some time to rest his arm a little.
"He didn't pitch at all last year [because of hip surgery], and he's a guy we talked about maintaining his innings a little," Wakamatsu said. "The plan is to give him a start on the 20th down in Tacoma and reevaluate at that point, on whether he will come back and start [on July 25]. We'll see.
"A lot can happen."
Vargas, who is 3-3 with a 3.82 ERA, pitched five scoreless innings against the Orioles on Wednesday afternoon but needed 98 pitches to get 15 outs.
Vargas needs to make adjustments that allow him to get outs earlier in the count and take some of the pressure of the bullpen.