Kline became involved in theater and made his professional debut in 1971 as part of the Lincoln Center Repertory Company. Regional theater productions during this period included Chemin de Fer (in Chicago with actor Dennis Franz), Death of a Salesman, and Love's Labour's Lost. A classically trained singer, Kline made his Broadway career debut in the 1989 musical City of Angels.
On Three's Company, Kline played Larry Dallas, a playboy neighbor, used car salesman, and best friend to John Ritter's Jack Tripper.
In addition to his television and big screen appearances, Kline has hosted two game show pilots, Jumble in 1988 and To Tell the Truth in 1990.
In February 2010, he was cast as the Wizard in the first national tour of Wicked.
Kline was seen next in October 2011 in It Shoulda Been You, directed by David Hyde Pierce and starring Tyne Daly. In 2016, Kline appeared in a movie directed by and starring Mike Birbiglia called Don't Think Twice.
Kline played Kid Twist in the Paper Mill Playhouse world premiere production of The Sting starring Harry Connick Jr. which ran April 8–29, 2018. In December 2018, he joined the national tour of the musical Waitress through August 2019. He reprised the role in the Broadway cast from August 20, 2019, until the show closed on January 5, 2020.