Zellner attended Mary Persons High School, where he lettered in football and basketball. In football, he was an All-Area selection and an All-District selection, and Prep Football Report named him as an All-Dixie selection. After his senior season, he participated in the Florida-Georgia All-Star Game. In basketball, he was an All-Area selection and played in the North-South All-Star tournament. Although he received a scholarship from the University of Georgia, academic problems forced him to join Georgia Military College. He began as a strong-side linebacker and was moved to the defensive line as a sophomore. He registered 129 tackles and 15 sacks in his two junior college seasons. In 1996, he transferred to Fort Valley State University, but eligibility problems limited his playing time to only 3 football games, collecting 18 tackles. The next year, he focused on basketball, starting at forward, averaging 10.1 points and 9.3 rebounds, while helping the school win its first conference championship since 1969. In 1998, he returned to play football at defensive end, posting 60 tackles, 14 tackles for loss and 3 sacks. He received All-American and All-Southern Intercollegiate Athletic Conference honors.
Zellner was selected by the Dallas Cowboys in the fourth round of the 1999 NFL Draft, after impressing in the scouting combine, recording the second fastest 40-yard dash time, first in the broad jump and first in the 20-yard dash among defensive linemen. As a rookie, he had 10 special teams tackles. Against the Atlanta Falcons, he posted his first career sack, while adding a quarterback pressure and 2 tackles. In 2000, he played in 12 games, registering 13 special teams tackles, 16 defensive tackles, 2 sacks and 3 quarterback pressures. He missed the last 4 games, after being placed on the injured reserve list with a torn left anterior cruciate ligament he suffered during a practice. In 2001, a season-ending back injury to Ebenezer Ekuban, allowed him to start 15 games at right defensive end, setting career-highs with 46 tackles, 3 sacks and 22 quarterback pressures. The next year, he only started 2 games and registered 29 tackles. He wasn't re-signed at the end of the season.
Washington Redskins
On May 12, 2003, he was signed as a free agent by the Washington Redskins to a one-year contract. He recorded 14 tackles and one sack as a backup during the season.
The Oakland Raiders signed him as a free agent in 2004, but in less than a month he was traded to the Arizona Cardinals along with Troy Hambrick, in exchange for a sixth round draft choice in the 2005 NFL Draft.
Arizona Cardinals
Zellner replaced an injured Fred Wakefield, playing in all 16 games in 2004, while registering 35 tackles and 2 sacks. He was re-signed at the end of the year, only to be released on August 29, 2005.
Personal life
In 2002, Zellner was charged with drug possession, however, the charges were later dropped. He is a cousin of safetyTra Battle who also played for the Dallas Cowboys.
