Seattle announced the contract Monday after the first baseman completed a physical. The agreement includes options for 2026-28 and likely will cover all the years White would be eligible for salary arbitration.
"I believe in the organization and where we're going and it's something I want to be a part of," White said.
The 23-year-old was the 17th overall pick in the 2017 amateur draft out of Kentucky. From the time he joined the Mariners' system he's been regarded as the club's first baseman of the future because of his excellent defense and a solid bat at the plate.
White's deal with the Mariners can max out in the mid-$50 million range, sources told ESPN's Jeff Passan.
"For us to be able to do this with a player who has yet to play in the big leagues but we have become so comfortable with, not just with what we believe his upside as a player to be, but who he is as a person, I think all of that has gone into making this a very easy thing for us organizationally," Seattle general manager Jerry Dipoto said.
Philadelphia infielder Scott Kingery ($24 million for six years) and Chicago White Sox outfielder Eloy Jimenez ($43 million for six years) agreed in March to lucrative contracts before their big-league debuts, but both had played at Triple-A.
It's the largest contract for an American player who has not played above Double-A, and Seattle added White to its 40-man roster in order to sign him to a big league deal. White said the first conversations started a little over a week ago.
"I didn't take this lightly. It's something took a lot of time thinking about and I think this is the best thing for us," White said.
Seattle envisions White as a major part of its future core, along with fellow top prospects Jarred Kelenic, Kyle Lewis, Logan Gilbert, Justin Dunn and Justus Sheffield. White is the only player in the organization currently under contract beyond the 2021 season. Both Kyle Seager and Yusei Kikuchi have options beyond 2021 but neither is locked in like White.
White should have a chance to earn the starting job at first base -- the Mariners used eight players there last year. Austin Nola and Daniel Vogelbach had the most games at first, but Seattle thinks White is a better defender.
While White's defense has been highly regarded, his bat isn't bad, either. He hit .293 with 18 homers and 55 RBI at Double-A Arkansas last season. White has hit .296 with an .832 OPS in parts of three seasons in the minors.
The Associated Press contributed to this report.