Thiessen founded Relient K in 1998, with guitarist Matt Hoopes, and bassist Brian Pittman,[1] and recorded the demo All Work & No Play the same year.

[3] In 2007, Gotee, Capitol, and EMI released Relient K's fifth studio album, Five Score and Seven Years Ago, which peaked at number six on the Billboard 200.

[7] Although he had previously expressed interest in recording a studio release, he had placed his priority on Relient K, and relegated his solo work to his free time.

In 2009, he contributed vocals, songwriting, and production to Owl City's major-label debut Ocean Eyes, which has sold over 1,500,000 copies worldwide and reached the top ten on the Billboard 200.

[11] Lead single "Fireflies" reached number one on the Billboard Hot 100 in late October, and was a global success, topping the charts in many countries including Australia, Denmark, the Netherlands, and the United Kingdom.

[12] Thiessen continues to work with Owl City's Adam Young and contributes vocals to "Plant Life", a song on the album, "All Things Bright and Beautiful".

[citation needed] Thiessen has also made guest contributions on a number of releases with other artists, including Blackbear, John Reuben, tobyMac and The Fold.

[citation needed] Thiessen has received piano lessons since he was seven years old, with early interests in ragtime and Scott Joplin, specifically the soundtrack from The Sting.

[13] Since Relient K's early days, Thiessen's main influences have been in the pop punk genre,[6] specifically NOFX, Less Than Jake, Ghoti Hook, MxPx,[5] and Goldfinger.

Later, his mother remarried and the family moved to Bolivar, Ohio, where he met and befriended future bandmates Matt Hoopes and Brian Pittman.

[16] Thiessen dated future pop star Katy Perry for a time from 2003 until 2005, and wrote several songs with her and Glen Ballard during her early recording sessions.