Karl originated the roles of Kyle in Legally Blonde (2007), Joe in 9 to 5 (2009), and Edward Lewis in Pretty Woman: The Musical (2018).

[5] Karl originally planned on becoming a voice teacher until the director of a show he was in at college told him he had what it took to be a professional actor.

In February 2000 Karl was an understudy in the New York premiere of Stephen Sondheim's Saturday Night at the Second Stage Theatre.

[9] Karl was next seen as Dino Del Canto in the world premiere of the musical Romeo and Bernadette at the Coconut Grove Playhouse in Miami, Florida.

[12] Karl was also seen at the Paper Mill Playhouse in June and July of that year, when he performed as Danny Zuko in Grease.

In January 2004 he performed in the musical revue Nights on Broadway II at Caesar's Palace in Atlantic City, New Jersey.

[13] In September 2004, Karl played Luke in the premiere of Altar Boyz at the New York Musical Theatre Festival.

[15] Karl's next role was the title character Adam Patterson in the Off-Broadway musical Slut at the American Theatre of Actors in Fall 2005.

[17] He then joined the Broadway cast of The Wedding Singer at the Al Hirschfeld Theatre, where he played the roles of Drunk Dave and Bad Haircut Guy until the show closed at the end of 2006.

Karl's next role was in Legally Blonde, where he played Kyle "the UPS guy", Dewey, and Grandmaster Chad at the Golden Gate Theatre in San Francisco, where the musical had its pre-Broadway tryout from January 23 to February 24, 2007.

[24] Karl played Neville Landless in the Broadway revival of The Mystery of Edwin Drood which opened on November 29, 2012, at Studio 54.

[27] In order to get into shape to play Rocky Balboa, Karl put on 12 pounds of muscle and worked with a trainer to get to a heavyweight fighter's weight.

On August 15, 2017, the producers of Groundhog Day announced it would end its Broadway run on September 17, 2017, but that a national tour and a possible return to London would follow.

[33] In May 2018, it was announced that Karl would star as Edward Lewis in the Broadway production of Pretty Woman: The Musical, following Steve Kazee's departure.

[36] On August 12, 2022, it was announced Karl would play the roles of the Big Bad Wolf and Cinderella's Prince from September 6-15 in the Broadway revival of Into the Woods filling in for Gavin Creel at the St. James Theatre.

[39] He reprised the role of Rapunzel's Prince during the opening weekend of the U.S. national tour's engagement in Washington, D.C., at the Kennedy Center this time filling in for Jason Forbach who went on for the Baker in the place of Sebastian Arcelus.