SEATTLE -- For a day, Alex Jackson was a big leaguer.
Wearing a blue Mariners pregame jersey with his name and No. 10 inscribed on the back, Jackson made small talk with a few assistant coaches and catcher Mike Zunino. During his batting practice session, he sprayed line drives into Safeco Field's spacious power alleys.
A product of San Diego's Rancho Bernardo High School, Jackson, 18, is an "advanced high school hitter" with power from the right side of the plate, according to Mariners director of scouting Tom McNamara.
It's the primary reason the Mariners selected him earlier this month with the sixth overall pick in the 2014 First-Year Player Draft. The club made Jackson's signing official Monday a few hours before it began a three-game series against the Red Sox.
His signing bonus was a reported $4.2 million.
"I mean, it's been unbelievable," Jackson said of his introduction. "It's one of those experiences where you can only have once in your life, besides down the road playing for an MLB team. It's definitely an honor. I was very glad I was able to do it."
Jackson is 6-foot-2 and weighs 215 pounds. He primarily played catcher in high school but will begin his professional career as a right fielder when he reports to Peoria, Ariz., on Tuesday to play for the Rookie-level Arizona League Mariners.
Gary Patchett, a Mariners scout based in Southern California, ran point on Jackson. He said the 2014 Perfect Game First-Team All-America selection shouldn't have a problem adjusting to a new position.
"He played right field in the summer. He's an athlete. He's a guy that you can pretty much [put] anywhere and he's going to learn how to play it and be serviceable," Patchett said. "It's the bat that we were after, and we know that he's a premier right-handed hitter."
During his senior season with Rancho Bernardo, Jackson batted .400 (40-for-100) with a 1.459 OPS, 45 runs, seven doubles, four triples, 11 home runs and 31 RBIs in 35 games. Entering the Draft, Baseball America rated him the top available position player. He was also named to the California All-Region First Team.
According to Patchett, Jackson's a player that "eats, sleeps and breathes baseball."
General manager Jack Zduriencik agreed.
"I think we're all very excited. I think when you look at his desire to want to play ... you get a chance to meet his mom and dad, you know the cloth that he's cut from, which is impressive," Zduriencik said.
Jackson's high school numbers would be hard to duplicate in a video game set on "easy mode." In four seasons, he totaled 135 games while posting a .375 batting average with 156 runs scored, 35 doubles, six triples, 47 home runs and 127 RBIs. He left the prep ranks tied for the San Diego section career home run record with former Indians prospect Johnny Drennen (2002-05).
Jackson signed a letter of intent to play baseball for the University of Oregon, but he opted to instead sign with Seattle.
"It's a tough decision," Jackson said. "It's two big opportunities in my life and I really think I ended up making the best decision. I'm in a happy place and I'm glad I'm here so I think everything went well."
Zduriencik thinks Jackson will move quickly through the Minor League system, though he didn't want to speculate about a timetable for reaching the Major Leagues.
"We'll just let him play," Zduriencik said. "And his performances will dictate how quickly he moves, but right now we'll let him get his feet wet in Arizona and we'll see what happens from here."
Adam Lewis is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.