Lageman was born in Fairfax, Virginia. He attended Park View High School in nearby Sterling. He attended the University of Virginia for college, where he played both linebacker and defensive end for the Virginia Cavaliers football team.
Although considered undersized to play defensive end in the NFL, a fact that was pointed out by Mel Kiper Jr. during ESPN's coverage of the 1989 NFL Draft, Lageman was taken in the first round by the New York Jets. Lageman's NFL career spanned 10 seasons, the first six with the Jets, and the final four with the Jacksonville Jaguars. His best year came in 1991 when he made ten quarterback sacks. He made a career total of 47 sacks in the NFL.
In 1995 Lageman was signed by the Jacksonville Jaguars in their inaugural year. With Lageman playing a key role, the Jaguars made the 1996 playoffs and upset the Buffalo Bills and the Denver Broncos on the road before losing to the New England Patriots in the AFC Championship Game.
Lageman's playing career ended on the first game of the 1998 season in Chicago. Although the Jaguars won on a last second touchdown pass from Mark Brunell to Jimmy Smith, they lost Lageman to a torn muscle in his arm. He never played again.
During his time in Jacksonville, Lageman, an avid hunter and fisherman, began his broadcasting career with a radio show titled "The Outdoors Show" along with Captain Kevin Faver and Captain Kirk Waltz. After his playing career, Lageman continued his broadcasting career as a football color analyst on Fox television.