Leonhard was signed by the Buffalo Bills as an undrafted free agent in 2005 after playing college football at the University of Wisconsin.

During his freshman year, 1997, the Flambeau Falcons won the Wisconsin state championship in football in division six over River Ridge high school.

Leonhard was a walk-on football player at the University of Wisconsin–Madison, having received no Division I-A scholarship offers out of high school.

[2] He was not awarded a scholarship until his senior year, despite the fact that he had been a regular starting safety before that season and had already been named All-Big Ten twice.

He held the Big Ten Conference career punt return yardage record with 1,347 yards[4] until Michigan's Steve Breaston eclipsed his mark in 2006.

In 2007, Leonhard stepped in for injured starter Ko Simpson and recorded 13 tackles on opening day against the Denver Broncos.

In 2008, Leonhard signed with the Baltimore Ravens, where he started in 13 of 16 regular season games,[7] replacing the injured Dawan Landry.

During a game against the Kansas City Chiefs on December 11, 2011, Leonhard landed awkwardly after catching an interception thrown by Tyler Palko.

[21] In February 2017, Leonhard was named the new defensive coordinator of the Badgers, replacing Justin Wilcox, who became the new head coach of California.

[22] In late November 2017, Leonhard was named one of five finalists for the Broyles Award which recognized the top assistant coach in college football.