The 6-foot-2, 175-pound Mier played at Bonita High School in La Verne, Calif. He was committed to play for Southern Cal. Mier, 18, hit .394 with 28 hits, 18 RBIs and five home runs during his senior year of high school.
Astros scouting director Bobby Heck said Mier was the best player in the draft.
"We were elated he was still there," Heck said. "Picking where we did, we had to wait for some stuff to fall to us. It was an excited room once he did."
Mier participated in the Astros' pre-draft camp Friday at Minute Maid Park. Mier impressed the Astros' scouts with his maturity.
"I walked up and talked to him for a while," said general manager Ed Wade. "You could see the presence Bobby was talking about. You could see why they're excited about the prospects of getting him here and having a frontline, everyday shortstop for a long period of time."
Mier's older brother, Jessie, is a catcher in the Dodgers minor league system. Heck shied away from comparing Mier to any current major leaguers, but said he has the potential to develop into an everyday starter.
"There is a long body of work on him," Heck said. "He's a guy we really wanted."
Mier is the 11th infielder that the Astros have taken with their first pick. They've selected 19 pitchers, eight catchers and seven outfielders with their previous first picks.
Last year's first-round pick, catcher Jason Castro, was promoted to Double-A Corpus Christi on Monday.