He spent his entire eight-year NFL playing career with the Bears. Plank was the first Bears rookie to lead the team in tackles. Plank was a favorite of Bears defensive coordinator Buddy Ryan for his hard hitting and aggressive style, so that he named his defense the "46 defense" after Plank's jersey number. Plank was considered one of the hardest hitting safeties in the game. That effort took a physical toll and he retired before the Bears reached their peak in 1985. Plank and Gary Fencik were dubbed "The Hit Men", a fact referenced by Fencik in 1985's The Super Bowl Shuffle.
After football, Plank became a franchisee of Burger King Corporation operating multiple restaurants. In 1995 Plank began working as a football analyst and has done work for Fox Sports, Arizona Cardinals, Arizona State Sun Devils, University of Arizona Wildcats, Arizona Rattlers. In 1996, a Bears fan "Bearman" became the unofficial mascot of the Bears, and he wore Plank's 46 jersey. Since 2001, Plank has worked as a football color analyst on national radio broadcasts for Sports USA Radio and Westwood One. Plank is currently a voter in the weekly Harris Interactive College Football Poll ranking the top 25 college teams.
In 2001, Plank began his coaching career as a defensive coordinator in the Arena Football League for three seasons under former Dallas quarterback Danny White. In those three seasons, the Arizona Rattlers played in three consecutive Arenabowls. In 2004, Plank was hired by Arthur Blank to be head coach of the Georgia Force, an Arena Football team he owned in addition to the Atlanta Falcons. Plank was named the AFL's Coach of the Year in 2005 and 2007, leading Georgia to the playoffs in every season and in his first year, ArenaBowl XIX in 2005. In Plank's first four years as an AFL head coach, he won more games in that period than any other coach in the history of the AFL. In 2008, he was a seasonal assistant on the Atlanta Falcons staff. The Falcons played in the Wild Card round of the NFL playoffs. In 2009 he served as the assistant defensive backfield coach for the New York Jets under head coach Rex Ryan, the son of Plank's former defensive coordinator, Buddy Ryan. The 2009 Jets defense led the NFL in fewest total yards allowed, fewest points allowed, and fewest TD passes allowed. The Jets played in the AFC championship game versus the Colts. In 2010, Plank became a football program assistant at Ohio State. The Buckeyes earned a share of the Big 10 title with an 11–1 record and beat Arkansas in the Sugar Bowl. On August 31, 2011, Plank became head coach of the AFL's Philadelphia Soul. In Plank's first year, the Soul compiled a regular season record of 15–3 after going 6–12 the previous year. The Soul played in ArenaBowl XXV versus the Arizona Rattlers. The Soul established new franchise records in wins, scoring, rushing, and defensive takeaways in 2012.