Manager David Bell gave Mahle the news on Wednesday morning ahead of his second spring training start.
"It means a lot," Mahle said. "Some people never get to do this. I may only get to do it once. I don't know so, I will try to make the most of it and have a lot of fun."
A four-year veteran, Mahle, 27, tied for the major league lead with 33 starts last season, finishing with a team-best 13-6 record with a 3.75 ERA.
"It was a real honor to give him the news," Bell said. "He doesn't always show it, but he was really excited. He knows how important it is, and he's earned it. He has quietly become one of the best pitchers in baseball."
Luis Castillo made the Opening Day start last season and in two of the past three. He has shoulder pain and is expected to miss at least his first start of the season.
"I was surprised to be named because Castillo has been our guy for the last couple years," Mahle said. "He still is our guy, but I'm really excited."
Mahle has made 94 big league starts with the Reds and has a 26-31 career record.
"The last couple seasons I took a big step from the two previous seasons," Mahle said. "I was able to compete on a more consistent basis."
Also Wednesday, the Reds signed right-hander Hunter Strickland. The eight-year veteran made 57 relief appearances with the Rays, Angels and Brewers last season.
"He's going to pitch on the back end of our bullpen," Bell said.
Third baseman Mike Moustakas left Tuesday's game with a sore left shoulder after diving for a groundball. Bell said Moustakas will be held out of games until Saturday.