Bobby Heck, the director of scouting, and team general manager Ed Wade made the announcement after Mier passed a physical earlier in the day and presented the 18-year-old shortstop with a jersey during a news conference at Minute Maid Park.
"It's an exciting time, and I'm really happy to now be a part of the Astros organization," Mier said. "I'm excited to get out there and start my career. I'm going to continue to work hard and do whatever."
Mier spoke to MLB.com on Wednesday and said the deal was in place then pending him passing the physical. Although the terms of the contract were not disclosed, it is reported that he will receive a $1.358 million signing bonus.
Mier, who was picked No. 21 overall, flew into Houston on Thursday with his mother and brother and took batting practice with the Astros prior to the team's series opener with the Detroit Tigers.
"I'm going to get a thrill out of meeting all the players," Mier said. "The guys I've heard about are great guys and it's going to be a thrill to meet all of them. They are all big-name players."
When going out to stretch with the Astros during their pregame routine, left fielder Carlos Lee joked with the wide-eyed Mier, asking manager Cecil Cooper, "Who is he?"
Mier, who had previously committed to play at USC before signing professionally, will report to the Astros' Rookie League affiliate in Greeneville, Tenn., on Saturday.
Heck and Wade were happy they were able to sign Mier quickly and are eager to get him on the field to begin his career.
"The fact that he signed so soon gets him into the [developmental] environment that much faster," Wade said. "We have three first-year players playing in Double-A, including our first-round pick last year and our 48th-round pick last year. These months that [Mier] has now given himself an opportunity to take advantage of will hopefully accelerate his process and get him on the express lane."
The 6-foot-2, 175-pound shortstop hit .394 with 10 doubles, five home runs, 18 RBIs and 22 walks and had a .545 on-base percentage in his senior season at Bonita High School in Laverne, Calif.
"When we saw him at showcases, we saw the tools that you look for in a shortstop and he had all the actions that you look for," said Doug Deutsch, the Astros scout who evaluated Mier throughout his amateur career. "The ease with which he plays shortstop, for me, was very positive. It's just not a big effort for him."
Jason Grodsky is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.