The Chargers are expected to send a 2022 second-round draft pick and a 2023 sixth-rounder to the Bears in return for the 31-year-old Mack, a source said.
The deal marks the first major move for new Bears general manager Ryan Poles, who was hired in January. It also reunites Mack, the Defensive Player of the Year in 2016, with Chargers coach Brandon Staley. Staley was Mack's linebackers coach when the Bears acquired him from the Raiders in September 2018.
In that first season with Chicago, Mack finished with 12.5 sacks and in second place in the AP Defensive Player of the Year vote. It was the only double-digit-sack season he had in Chicago; he had six in 2021, but they came in seven games, as he had season-ending foot surgery in November.
The Chargers were in need of help along the defensive line. They finished 30th in rushing defense in 2021, allowing 138.9 yards per game, and allowed the highest conversion rate on third down in the NFL (69%).
Now, Mack will join four-time Pro Bowler Joey Bosa in what will be a fierce combination in Los Angeles. Over the past five seasons, Mack and Bosa both rank top four in total defensive pressures. They also both have exactly 12 strip sacks over the past five seasons, which is tied for sixth in the NFL.
Mack has three more seasons remaining on his contract and is owed $17.75 million in 2022, $22.9 million in '23 and $23.25 million in '24.
The Chargers will take on his full contract, a source told Schefter.
Los Angeles, 9-8 last season, got off to a 4-1 start but missed the playoffs for the third straight season, winning back-to-back games only once after Week 5, and losing three of the final four games.
When Staley was hired as Chargers head coach last January, he credited working with Mack for helping tremendously in his development as a coach.
"I learned a lot more from Khalil Mack than he learned from me,'' Staley said at the time. "I drew a lot of confidence being able to coach a guy like him and I think that that first season in Chicago and that first experience, I felt like, you know, I could do this someday.''
The Bears opted for major changes after going 6-11 and missing the playoffs for the ninth time in 11 years. They fired general manager Ryan Pace and coach Matt Nagy, replacing them with Poles and Matt Eberflus, respectively.
Trading Mack for draft capital is the first big step by the new regime toward overhauling the roster.
Pace's acquisition of Mack in a blockbuster trade with the Raiders -- Mack had a contract dispute with the team -- just before the start of the 2018 season helped transform a solid defense into an elite unit and propelled the Bears to the NFC North championship in Nagy's first year.
Mack was an All-Pro that season with 12.5 sacks, helping Chicago go 12-4 and make the playoffs for the first time since 2010. But the Bears lost a wild-card game at Soldier Field to Philadelphia. Chicago never won a playoff game with Mack.
Information from ESPN Stats & Information and The Associated Press was used in this report.