Johannsen attended Iowa State University in the early 1980s, originally majoring in veterinary medicine, and then later changing to chemical engineering. He left after three years in college and relocated to San Francisco, California in order to pursue a career in comedy. Johannsen made his debut at Cobbs Comedy Club in San Francisco, and by 1986 had won the 11th Annual San Francisco Stand up Comedian Competition.
During the 1990s, Johannsen starred in his own HBO Comedy Special, This'll Take About an Hour, which received rave reviews and was listed as one of the Ten Best Television Shows by People Magazine in 1992. In 1994, he was nominated as "Best Male Stand-Up Comedian" for the 1994 American Comedy Awards.
Johannsen is a favorite of David Letterman and has appeared for a record 32 stand-up performances on The Late Show with David Letterman, and his routines are heard daily on XM Radio's Channel 151 Laugh USA. He is married to actress Belinda Waymouth, with whom he has one child.