Blake Beavan, an 11-game winner last season, earned the other remaining starting spot and will pitch fifth in the rotation, opening against the White Sox in Chicago on April 5.
Veterans Felix Hernandez, Hisashi Iwakuma and Joe Saunders had already been announced as the first three starters when the Mariners open the season on Monday in Oakland.
The 6-foot-5 Maurer earned his spot with an outstanding spring, giving up just two runs over 20 innings of Cactus League play for a 0.90 ERA with 22 strikeouts and six walks.
"It's been a good ride," an elated Maurer said of his climb from a 23rd-round Draft pick in 2008. "I've learned so much every year, turning myself into a pitcher and not just a thrower. This year I've actually felt like more of a pitcher than ever, using a two-seam and throwing changeups in fastball counts and being able to throw a slider behind or ahead in the count, different types of sliders. Just going out there and learning how to pitch."
Maurer is the first Mariners pitcher to jump from Double-A to a starting spot with the Major League club since Mike Hampton and John Cummings both made the leap in 1993.
"He had a great camp," said manager Eric Wedge. "You're talking about a young man that has multiple plus pitches. The way it worked out, he faced a lot of big league lineups and handled it well. Different situations, he handled all of them. He showed great stuff, poise, and had some presence out there. He came in here and took it."
Maurer received word from Wedge and pitching coach Carl Willis in a brief meeting Wednesday, then immediately called his parents in Costa Mesa, Calif., to pass along the news.
"My mom was getting a little choked up and my dad was just rambling on about random stuff," Maurer said. "But it was good."
Maurer then celebrated briefly with some of the Minor League teammates that he's lived with and shared time with all spring, including fellow pitching prospects Taijuan Walker, Danny Hultzen and James Paxton, all of whom were expected to get to the Majors ahead of Maurer.
"We're always rooting each other on," Maurer said. "We have been this whole time and we still will. It's awesome to have a good group of friends like that to just rally behind you."
Maurer will pitch the Mariners' final spring game on Saturday in a 12:05 p.m. PT exhibition against the Rockies in Salt Lake City, putting him in position to make his Major League debut five days later. Beavan will stay behind and pitch a Minor League game on Sunday in Peoria in order to stay on schedule to face the White Sox on April 5.
Beavan, 23, pitched most of last year with the Mariners, but said he was more nervous this spring about making the club, given the competition brought in with Bonderman and Jon Garland as well as the crop of young prospects in camp.
But Beavan also said he feels he's pitching better than a year ago, as he continues moving ahead with a revised delivery that gives him more downward motion from his 6-foot-7 frame.
"One thing about Blake, he's never satisfied," Wedge said. "He's always working to get better and he finds a way to win ballgames. He's already gained some great experience in his young career. That, and some of the adjustments he's made and continues to make, are why he's on this ballclub."
Ramirez pitched three controlled innings in a Minor League intrasquad game on Wednesday morning, but clearly isn't ready to pitch a full game after missing some time last week with minor soreness in his arm. He was limited to five outs with two hits, three walks and three strikeouts against a group of Mariners Minor League hitters.
He'll begin the season instead with Triple-A Tacoma, which is where Bonderman will also likely pitch if he accepts his Minor League assignment.
Bonderman, 30, is attempting to return to the Majors for the first time since 2010, due to extensive arm and shoulder problems. He was told the news on Wednesday morning and will be given a day or two to decide whether to continue his comeback in the Minors or announce his retirement.
The moves put the Mariners roster at 30, with the only real decision still pending in regards to the fifth and final outfield spot between Jason Bay and Casper Wells.