The 21-year-old right-hander, rated the fifth-best prospect in baseball, yielded two hits and one run with two strikeouts and a walk. If the 6-foot-4 Walker (1-0) had any rookie nerves, it certainly didn't show as he retired the first eight batters he faced.
The Astros were up by one in the fourth before Ackley tied it with an RBI single. Seattle led by two in the fifth before a single by Ackley made it 5-1. A run-scoring single in the ninth gave Ackley four RBIs to tie a career best.
Houston starter Brad Peacock (3-5) was tagged for seven hits and five runs in 4 2-3 innings.
Abraham Almonte, also playing his first major league game, added two RBIs to help the Mariners win their second straight following a six-game slide.
Seattle relievers Carter Capps, Charlie Furbush and Oliver Perez combined to pitch four scoreless innings after Walker left the game.
Walker relied heavily on a fastball that was routinely clocked at 95 mph, but reached as high as 97 mph. He also used a cutter, curveball and slider to keep the Astros off balance.
A first-round draft pick in 2010, Walker joined the Mariners after going 4-7 with a 2.46 ERA in 14 starts at Double-A and 5-3 with a 3.61 ERA over 11 starts in Triple-A. He was dominant in his last two outings, going 2-0 with a sparkling 0.82 ERA.
Walker became the youngest starting pitcher to make his debut for the Mariners since Felix Hernandez's first game in 2005 at 19. The Mariners offset Walker's inexperience by starting catcher Henry Blanco, who turned 42 on Thursday. Manager Eric Wedge thought Blanco's presence would help make Walker feel more comfortable in his first game.
He was rolling before a tough third inning for Seattle's defense. Houston's first hit came with two outs on a double by Jonathan Villar that bounced off the glove of left fielder Raul Ibanez.
An error by first baseman Justin Smoak allowed Robbie Grossman to reach safely as Villar scored to give Houston a 1-0 lead.
L.J. Hoes singled on a grounder that deflected off Kyle Seager's glove at third base. But Walker didn't get rattled, and after a short visit to the mound from Blanco, he retired Jose Altuve to limit the damage.
Walker set down six of his last seven batters, with the only baserunner coming on a walk to Brett Wallace with two outs in the fourth.
Walker was helped by some dazzling defense for the last out of the fifth. Ackley sprinted before sliding on his knees to grab a fly ball hit by Grossman near the bullpen in right-center. Walker pumped his fist and smiled before leaving the mound.
Walker's debut came in front of about 40 family members and friends who traveled to Houston from California and Louisiana. Several members of the group, which included his mother, father and nine of his 11 siblings, donned his pinstriped No. 44 jersey from the Double-A Jackson Generals. Walker wore No. 27 on Friday night.
His mother, Nellie Garcia, was emotional as she watched her son pitch. The two shared an embrace shortly before he took the mound.
"I think it was one of the best moments of my life," she said of the hug. "We've had a lot of moments hugging on the field don't make me cry right now. I think that was the realest moment ever."
Walker's father, Frank, smiled broadly as he talked about his son and the opportunity to share this milestone with him.
"It is very exciting," he said. "It's awesome. It's unbelievable. Words really can't describe it."
Kendrys Morales walked to start the fourth and Smoak drew a one-out walk. The Mariners tied it on an RBI single by Ackley before taking the lead when Almonte grounded into a forceout that scored Smoak.
Brad Miller singled to start the fifth and stole second. Seattle pushed the lead to 3-1 on an RBI single by Seager.
He stole second and advanced to third on a wild pitch. Ackley's two-run single to center extended the lead to 5-1.
NOTES: The Mariners optioned RHP Erasmo Ramirez to Triple-A Tacoma and selected Almonte's contract. Almonte started in right field. ... The series continues Saturday when Seattle LHP Joe Saunders opposes Dallas Keuchel. ... Houston C Carlos Corporan, on the seven-day concussion list, said he no longer has concussion symptoms and is "back to himself." He has been doing individual work, but isn't sure when he'll return. ... Olympic gold medalist and softball star Cat Osterman threw out the ceremonial first pitch.