Root was born in Sarasota, Florida, the son of Leona Estelle and Rolland Clair Root, a construction supervisor. Root has likened his childhood to that of an "Army brat", as his family frequently relocated while his father finished each construction project. Root graduated from Vero Beach High School in Vero Beach, Florida. He received his AA from the University of Florida, and trained in the BFA acting program. The university recognized him as one of its distinguished alumni in 2008.
Among his most recognized television roles are eccentric billionaire Jimmy James on the sitcom NewsRadio and as the voice of divorced, down-and-out Army barber Bill Dauterive and Hank Hill's hedonistic boss Buck Strickland on the FOX animated television series King of the Hill (Root originally auditioned for the role of cuckolded, conspiracy theorist and pest control worker Dale Gribble). He had a recurring role on the final two seasons of The West Wing as Republican campaign consultant Bob Mayer. His most recognizable film roles to date are as the mumbling, quirky Milton Waddams in Office Space, as the mild-mannered gymrat Gordon Pibb in Dodgeball: A True Underdog Story, as Suds, a drunk sports writer, in Leatherheads, and as the dimwitted principal in the 2008 Judd Apatow produced comedy Drillbit Taylor.
Root is a favorite of the Coen Brothers. He has appeared in O Brother, Where Art Thou?, The Ladykillers, and No Country For Old Men. Kevin Smith wrote a role in Jersey Girl specifically for Root.
Root reunited with Mike Judge in Idiocracy and has also portrayed NASA flight director Chris Kraft in the miniseries From the Earth to the Moon. He took on the biographical role of Richard A. Clarke in The Path to 9/11. He has also had many guest appearances in television programs across several genres. He was Klingon Captain K'Vada in the Star Trek: The Next Generation episode "Unification" in 1991. In 1992, Root appeared on the TV series Night Court as Mr. Willard. In the 19931994 television season, Root appeared as a series regular as "R.O." on the Beau Bridges/Lloyd Bridges comedy/western series Harts of the West on CBS.
Root guest starred on Seinfeld, playing the role of a bank manager in the episode "The Invitations", handling Kramer's hello greeting issue. He had a role in Frasier in the episode "Detour" as the father of the family helping Frasier when his car broke down. In the CSI episode "Homebodies", he played the father of a rape victim. In addition, Root had a series regular role on the short-lived CBS series Ladies Man, which starred Sharon Lawrence, soon after she left NYPD Blue. He played Raymond, a hunting store owner involved in illegal gun trading, in the 1993 In the Heat of the Night episode "A Love Lost".
Root also has an active career as a voice artist. Aside from his roles in King of the Hill, he has played various characters in animated films, such as Ice Age, Finding Nemo, and Courage the Cowardly Dog. In other animated television, series regular roles include: Commander Chode in Tripping the Rift and Homebase in The X's. Additionally, he has provided guest-starring vocal talent in animated television series, like Chowder and Teen Titans. He recently lent his voice to Batman: The Brave and the Bold as Woozy Winks and The Penguin.
Root has appeared in HBO's series True Blood as a vampire named Eddie. He also had a multi-episode arc in the second season of Pushing Daisies on ABC, playing the mysterious Dwight Dixon. He appeared as Johnny Forreals, inventor of the word "boo-yah" on an episode of Comedy Central's The Sarah Silverman Program entitled "Cangamangus". He appeared in the 2009 comedy film The Men Who Stare at Goats as Gus Lacey. In 2010, he appeared in a multi-episode arc in season 8 of the Fox television series 24 as a probation officer named Bill Prady, and in the FX television series Justified as the eccentric Judge Mike "The Hammer" Reardon. In 2011, he co-starred in Robert Redford's The Conspirator, playing John Lloyd, a key witness in the trial of alleged Lincoln conspirator Mary Surratt. In 2012 he began playing the role of Gaston Means in the HBO's acclaimed TV series Boardwalk Empire season 3.