Background and college
Born October 27, 1982 in Ladysmith, Wisconsin, Leonhard played his high school football at Flambeau High School in Tony, Wisconsin where he was the starting quarterback and free safety. During his freshman year in high school, the Flambeau Falcons won the Wisconsin state championship in football. Leonhard was a team captain in his junior and senior seasons, the same seasons in which he was a first team All-State performer. He was also named team MVP in his senior year.
Leonhard was a walk-on football player at the University of Wisconsin-Madison, having received no Division 1-A scholarship offers, and was not awarded a scholarship until his senior year, despite the fact that he had been a regular starting safety before that season. He totaled 21 interceptions, which tied Jamar Fletcher for the most in school history. He also made 281 tackles, 21st in team history. He was regarded as one of the best punt returners in the league and held the Big Ten Conference career punt return yardage record with 1,347 yards until Michigan's Steve Breaston eclipsed his mark in 2006. Leonhard was a basketball player until he started with football in Wisconsin. The next year, he was an All-American and the team's MVP.
Before the Jets
Leonhard was the only undrafted rookie on the 53-man opening day roster for the 2005 Buffalo Bills, playing in ten games. In 2006, he played in 15 games for the Bills and received his first career start, his only one that season, on December 10. In the 2007 season, Leonhard stepped in for injured starter Ko Simpson and recorded 13 tackles on opening day against the Denver Broncos. He would go on to record 51 tackles for the team despite being hampered by a calf injury halfway through the season. He was the team's sixth leading tackler. He also recorded 32 yards on two kickoff returns and 36 yards on 4 punt returns.
On April 28, 2008, Leonhard signed with the Baltimore Ravens. He started in 13 of 16 games played in the regular season, setting career-high marks with 85 tackles and one sack to go along with an interception. On November 30, 2008, Leonhard scored his first NFL touchdown in a 34–3 victory over the Cincinnati Bengals. In his first two NFL playoff games, Leonhard has recorded one interception with a 20-yard return, one forced fumble, one fumble recovery, a half-sack, one pass deflected, 11 tackles, and a 29-yard punt return.