Cust, 31, spent the past four years with the Oakland A's and will be counted on to add some left-handed power to the middle of a Seattle lineup that struggled to produce runs last year.
Cust signed a one-year, $2.5 million deal after passing a team physical. The club will wait until Monday to make a corresponding move to clear a spot on the 40-man roster.
The Mariners have also agreed to terms on a two-year contract with veteran catcher Miguel Olivo, but will not make that deal official until he passes a physical sometime next week.
Cust, 31, hit .272 with 13 home runs and 52 RBIs in 112 games with Oakland last season after opening the year in Triple-A. He's a career .245 batter with a healthy .378 on-base percentage thanks to his ability to draw walks.
His 377 walks since 2007 are the most in the American League in that time frame and his .381 on-base percentage ties for 10th in the AL, which is a significant factor for Mariners general manager Jack Zduriencik.
"We were looking obviously for something to help our offense. Jack fit the bill," Zduriencik said. "He's a guy with power, a left-handed hitter that we think will play well in Safeco Field. He's a guy that's been durable and we really like his plate discipline.
"He grinds out at-bats. It's a good thing when you're hitting behind him in the middle of the lineup as he will do for us, it extends a pitcher. His power and plate discipline are real positives."
Zduriencik said Cust will be viewed as the club's primary DH going into Spring Training.
"He can play first and left, but those are not his strengths," Zduriencik said. "This guy is a power hitter who can drive the baseball, and that's why he's here."
The 6-foot-1, 235-pounder has 102 career home runs. He averaged 28 home runs and 76 RBIs a season in a three-year span from 2007-09 before his at-bats dipped last year.
Cust has five home runs in 97 at-bats at Safeco Field in his career and said the expansive park doesn't bother him at all, having played in Oakland the last four seasons.
"Obviously any hitter wants to hit in little bandbox. That's on everyone's wish list," Cust said earlier this week after agreeing to terms with the Mariners. "But I hit 25-30 home runs in Oakland every year until last year. Hitting home runs is never a problem for me on any field."
Cust said he's willing to play in the outfield and noted it will be a pleasure to play with -- instead of against -- Gold Glove Award winners Franklin Gutierrez and Ichiro Suzuki.
"Those guys have both robbed me quite a bit," he said with a chuckle. "It'll be good to get away from those guys catching balls I've hit."
Cust has hit 60 of his 97 home runs since 2007 as a designated hitter, tied with Vladimir Guerrero for third most in the Major Leagues in that span behind Jim Thome  and David Ortiz .
Zduriencik said Milton Bradley will also play some DH as well as left field and the team is eager to see how he looks in Spring Training now that his knee is healthy.
Besides the addition of Olivo, the Mariners would like to add an infielder in the coming weeks, and Zduriencik said the possibility of a trade remains after discussions were begun at the Winter Meetings. But he said not to expect a lot more free-agent signings.
"Our ears are open. You're always having dialogue," he said. " I don't think we're going to be huge players in this. I know we're not. ... We had some limitations with where we're at with payroll and where we can go with this. But were working within our parameters and it's very fair.
"Last year had flexibility. Next year we'll have flexibility. This year, we are where we are."