ESPN's Adam Schefter previously reported the record-setting contract is worth $66 million.
Brown, 25, spent last season with the New England Patriots. He started all 16 games at the left tackle spot, taking over the spot formerly held by Nate Solder.
While Solder fell off in New York, Brown became yet another player who maximized his potential with the Patriots. After spending most of his career playing as a right tackle, Bill Belichick switched him over to the left side in 2018 and watched him blossom into a star.
"Trent's very athletic," Belichick said, per NESN's Doug Kyed. "A lot of times on the left side of the line you face some very athletic players. Some have size but a lot of them are maybe a little undersized relative to the guys on the right side, but their speed and athleticism can be a little bit greater.
"He's able to match up with those guys with his length and his athleticism, and then he has some advantages over there with his size and his power. He's a very unique player with his skill set. The size is rare. His athleticism is good but when you combine it with his size, that's rare too. You don't see guys that big playing over there and he's very gifted."
Brown was also brilliant during the Patriots' run to Super Bowl LIII, ranking as the top-rated tackle of the playoffs by Pro Football Focus metrics.
The only downside of the Patriots acquiring Brown when they did was his impending free agency. By waiting to ink him to a long-term contract, the Patriots put themselves in a bind.
Had the Patriots known what they were getting when they made the trade with the San Francisco 49ers, odds are they would have pushed for a below-market contract.
Instead, the Raiders swooped in with a massive deal the Patriots could not or would not match. So for the second straight offseason, New England will be searching for a new left tackle.
The Raiders, meanwhile, continue a rebuild of their offense via talent from other teams. They previously traded a third- and fifth-round pick to the Pittsburgh Steelers in exchange for Pro Bowl wideout Antonio Brown.
The 2019 season will clearly be a litmus test for Derek Carr, who struggled his first year under Jon Gruden. With a franchise left tackle and franchise wideout on the roster, the pressure is going to be on Carr to return to a Pro Bowl level before the Raiders' move to Las Vegas in 2020.