Mengden's start comes on the heels of the Athletics placing Rich Hill on the 15-day disabled list with a left groin strain.
"It's been fun, it's been really quick," Mengden said. "They told me just keep doing what I'm doing. Started out in Double-A, did pretty good, got the promotion to Triple-A. Last night, coach [Steve Scarsone] told me [I was] getting called up here. It's a dream come true. Everything I've wanted since I was a little kid."
Mengden, who was acquired by Oakland for Scott Kazmir from the Astros last summer, is rated as the No. 14 prospect in the Athletics' system by MLBPipeline.com. Many underrated Mengden because of a stress fracture in his back that he dealt with coming out of college at Texas A&M.
Mengden missed about two months with the stress fracture, which he suffered while throwing a pitch, and wasn't really completely healthy until getting to rehab it after the 2014 season. This offseason, Mengden was able to lift weights for the first time.
"This is my first offseason I was allowed to work out fully," Mengden said. "When I was originally drafted by the Astros, I had a stress fracture in my back, and it kind of limited me to mostly rest my first offseason. But this past one, I was really able to lift some weights, gain some good weight and felt really good coming into spring training."
Mengden brings a five-pitch repertoire to the table: a two-seam and a four-seam fastball, a curveball, a changeup and his best pitch, a cutter. That cutter, which Mengden said is normally his out pitch, hasn't always been a pitch Mengden has used.
When Mengden came out of Texas A&M, that cutter was actually a slider. When he got to the Minors, the baseball had lower seams, which changed how he threw the slider, ultimately morphing it into the cutter he uses now.
"The smaller laces and everything, just all of a sudden turned into a hard cutter and just kept throwing it," Mengden said.
While Athletics manager Bob Melvin has only seen his new starter pitch a couple of times in Minor League camp, he was left impressed.
"He had a good-moving fastball, a complement of pitches," Melvin said. "Looked like he competed real hard, had some velocity as far as sink as well."