Stearns, 30, will replace Doug Melvin, who is transitioning to an advisory role with the Brewers after serving as the team's GM since 2002.
At a news conference Monday, Stearns said his initial impression was that manager Craig Counsell is "an impressive guy" and he will take his time evaluating Counsell and the baseball operations department.
He was steadfast in his belief the Brewers could return to the playoffs for the first time since 2011, though he stopped short of offering a specific timeline.
"I would have not have come here if I didn't think it was possible to win a World Series in Milwaukee," Stearns said.
Stearns, a 2007 Harvard graduate, worked for the commissioner's office from 2008 to 2011 and spent two years as director of baseball operations for the Cleveland Indians before joining the Astros as the top assistant to GM Jeff Luhnow.
His duties in Houston included player evaluations and contract negotiations, and he also helped oversee the Astros' scouting, player-development and analytics departments.
"His experience is quite impressive," Brewers owner Mark Attanasio said Monday. "The fact that he was able achieve all that at a comparatively young age is ... testament to his work ethic and his drive."
Stearns will be the youngest current general manager in baseball, and he is one year younger than Milwaukee's most high-profile player, outfielder Ryan Braun.
"Trust me, I've had plenty of text messages over the last 24 hours that I will age tremendously," Stearns said with a smile Monday.
The Brewers, who faded badly down the stretch last year to miss out on a postseason berth, are 63-86 this season and last in the NL Central with two weeks to play.
FoxSports.com earlier reported Stearns' imminent hiring by the Brewers.
The Associated Press contributed to this report.