Geijerstam was born in 1946. His career in Swedish show business started with sound engineering in the early 1960s. In 1966 he became a member of the Swedish pop band Ola & The Janglers, one of the major rivals to Benny Andersson’s band The Hep Stars.
In 1973 Claes was a member of the duo act Nova who competed against ABBA in the Swedish selections for the Eurovision Song Contest. Nova’s contribution, You Are Summer, You Never Tell Me No, won the selections. ABBA’s Ring Ring only finished third.
Claes joined ABBA as a sound engineer on their 1974/75 tour of Europe, and the tour of Europe and Australia in 1977. He was also there to take care of the sound on the 1979/1980 tours of North America, Europe and Japan. As a performer he actually toured places like the United States before ABBA: he was there with his group Rocket as early as the mid-1970s.
Claes af Gejierstam became a television personality in the early 1970s, but is primarily well-known in Sweden as a popular DJ on radio and in discotheques. In the 21st Century he has served as one of the judges on Idol, the Swedish version of Pop Idol/American Idol. He has also toured Sweden as the compère of The Original ABBA Orchestra In Concert, featuring ABBA’s original session and touring musicians.