Dan Snyder took over the Commanders in 1999 and ran the franchise into the ground. It wasn’t until he brought back three-time Super Bowl winner, Joe Gibbs, in 2004 that they became respectable again. Gibbs returned the team to the playoffs but left because he was old and didn’t want the stress (via Washington Post).
But it’s challenging to choose the worst coach of the modern era because there are several terrible options. Jim Zorn didn’t know what he was doing while the less said about Marty Schottenheimer’s tenure, the better. Steve Spurrier tried to turn the Florida Gators into an NFL franchise and that didn’t work out.