RENTON, Wash. -- The Seattle Seahawks' Christmas present to Pete Carroll: a new contract that runs through the 2021 season.

The Seahawks signed the head coach to an extension a day after they secured a wild-card berth with their 38-31 win over the Kansas City Chiefs, which will make it seven playoff appearances in nine seasons under Carroll and general manager John Schneider.

"This will continue the championship culture that we have created in Seattle," Jody Allen, the team's chair and trustee of the Paul G. Allen Trust, said in a statement.

At 67, Carroll is the NFL's oldest head coach. The extension will take him through his 70th birthday.

Carroll told a small group of reporters after the team's announcement that the extension had been in the works since the organization approached him last week. It was wrapped up Monday afternoon following his morning news conference.

His previous extension ran through the 2019 season.

"The organization has been exceedingly good to me throughout the whole time we've been here, and they've continued to take that position, and I couldn't be happier," Carroll said.

It's Carroll's fourth contract with the Seahawks since he came to Seattle in 2010 after nine years at USC. Under Carroll, the Seahawks have gone 88-54-1 in the regular season and 9-5 in the playoffs, which includes the franchise's first Super Bowl championship after the 2013 season and another Super Bowl appearance the following year.

Carroll passed Mike Holmgren for the most wins in franchise history earlier this season. Holmgren is the franchise's longest-tenured head coach at 10 seasons, but Carroll's extension will carry him past that.

"So grateful to Jody and the organization," Carroll said in the announcement. "I love this team and couldn't be more proud to represent the 12s."

When Carroll signed his previous extension in the summer of 2016, he said there was no end to his coaching future in sight. By the end of last season, as the Seahawks were stumbling toward a 9-7 finish that left them out of the playoffs, speculation about Carroll possibly retiring had gained enough traction that he felt compelled to shoot it down, writing in a tweet: "People talking about retirement... I ain't old enough to think about retiring!"

Carroll and his wife adopted a plant-based diet in the summer of 2017 that he has described as life-changing.

"We have gotten rid of gluten, we have gotten rid of dairy products and just cleaned it up and found that all of the old arthritic problems have gone away," Carroll told ESPN in August. "Really, something in there in the diet has helped in terms of the inflammation. It's startlingly different. Over my lifetime I've taken Celebrex and Vioxx and more Advil than I'd like to admit and all that stuff. I don't take anything anymore, and I don't need anything anymore. The diet has really changed everything."

Speculation about Carroll's future after last season was tied to the major offseason personnel changes that appeared to be looming. The Seahawks moved on from franchise greats such as Richard Sherman and Michael Bennett while losing Kam Chancellor and Cliff Avril to injuries. Carroll also fired his offensive and defensive coordinators plus his offensive line coach as part of an overhaul of his coaching staff.

All the turnover was why outside expectations were so low for the Seahawks heading into 2018, with the most pessimistic projections giving them as few as four or five wins. Carroll said postgame that those doubts made their postseason berth all the more gratifying, a sentiment shared by several players.

"There's an emotion to it that's deep, and it's because there wasn't very many people that thought we could do this," Carroll said Sunday night. "Most everybody thought we didn't have a chance."

Seattle's seven playoff appearances since 2010 are the second-most of any NFL team in that span to the New England Patriots' nine, according to ESPN Stats & Information. The Seahawks made the playoffs only 10 times in the 34 seasons before Carroll and Schneider arrived.

"This has been a really, really fun season," Carroll said Sunday night. "I can't even believe that here we are with the last week of the season coming up. I can't believe that it has gone so fast. It has been as much fun, and it's because of the people. The whole mentality and the attitude, and we've had a blast doing this. And a great locker room, again. We've had locker rooms all year long that the guys have been celebrating with one another and caring for one another. It's a great part of a team game, it's why football is such an amazing game."

Jody Allen has assumed control of the team following the death of her brother, Paul Allen, the Microsoft co-founder who died in October due to complications from non-Hodgkin's lymphoma. She had final say on Carroll's extension.

The team is not expected to be sold anytime soon.

Schneider's contract reportedly also runs through the 2021 season. Said Carroll after signing his previous extension: "I don't want to do this without John."

"I'm not sure about the connection of the years and all that, but we've come here to do this together and he's been a great partner," Carroll said Monday when asked about his contact being concurrent with Schneider's. "We're both pumped up about everything that's coming. We love this team, we're excited about what's happened with ownership, and really it feels like we're just getting started. That's what it feels like."