Brown was initially drafted by the Pittsburgh Steelers in the second round, and became an important part of the "Blitzburgh" defense. Initially, he played at the inside linebacker position in the 3-4 with Levon Kirkland. He started at right inside linebacker in Super Bowl XXX for the Steelers, a loss to the Dallas Cowboys. He then moved to the outside when Greg Lloyd was lost for the 1996 season due to injury, where he went to his first Pro Bowl after that season. His success would continue with the Seattle Seahawks, going to two more Pro Bowls in the 1990s.
He signed with the Patriots as a replacement for injured inside linebacker Tedy Bruschi in 2005. After several games of what was thought to be subpar performance at his new position, Brown spent most of the rest of the season only on special teams. In September of 2006, after re-signing with the Patriots, Brown was released by the Patriots as one of their final cuts. He then signed with the Steelers and finished the season on their injured reserve.
On July 19, 2007, he re-joined the Patriots but was released on September 1, 2007. On September 11, 2007, the Pats re-signed him, and released him again on October 10, 2007. Brown was re-signed by the Patriots yet again on November 27, 2007, after linebacker Rosevelt Colvin was placed on season-ending injured reserve. He was released on December 27, 2007, before the Patriots became AFC champions for the season in Super Bowl XLII.
Since his NFL career began, Brown has also operated a business named Pro Exotics that sells non-venomous snakes.